[v13,41/45] sg: track lowest inactive and await indexes

Message ID 20210113224526.861000-42-dgilbert@interlog.com
State Superseded
Headers show
Series
  • Untitled series #93503
Related show

Commit Message

Douglas Gilbert Jan. 13, 2021, 10:45 p.m.
Use two integers in the sg_fd structure to track recent and lowest
xarray indexes that have become inactive or await a foreground
receive. This is used to shorten the number of xarray iterations
required prior to a match when queue (IO) depths are large, say
128.

Replace the req_cnt atomic in struct sg_fd with the inactives
atomic. With large queues cycles were wasted checking the request
xarray for any inactives when there were none to be found.

Rename the sg_rq_state_chg_*() functions to sg_rq_chg_state_*()
since too many things start with "sg_rq_state". Also the new
function names emphasize the change part a little more.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 drivers/scsi/sg.c | 304 ++++++++++++++++++++++++++--------------------
 1 file changed, 175 insertions(+), 129 deletions(-)

Patch

diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index bbf63787490b..2c9255d4abf1 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -116,6 +116,7 @@  enum sg_rq_state {	/* N.B. sg_rq_state_arr assumes SG_RS_AWAIT_RCV==2 */
 #define SG_FRQ_RECEIVING	7	/* guard against multiple receivers */
 #define SG_FRQ_BLK_PUT_REQ	8	/* set when blk_put_request() called */
 #define SG_FRQ_FOR_MMAP		9	/* request needs PAGE_SIZE elements */
+#define SG_FRQ_COUNT_ACTIVE	10	/* sfp->submitted + waiting active */
 
 /* Bit positions (flags) for sg_fd::ffd_bm bitmask follow */
 #define SG_FFD_FORCE_PACKID	0	/* receive only given pack_id/tag */
@@ -226,11 +227,12 @@  struct sg_fd {		/* holds the state of a file descriptor */
 	struct mutex f_mutex;	/* serialize ioctls on this fd */
 	int timeout;		/* defaults to SG_DEFAULT_TIMEOUT      */
 	int timeout_user;	/* defaults to SG_DEFAULT_TIMEOUT_USER */
-	int prev_used_idx;	/* previous used index */
+	int low_used_idx;	/* previous or lower used index */
+	int low_await_idx;	/* previous or lower await index */
 	u32 idx;		/* my index within parent's sfp_arr */
 	atomic_t submitted;	/* number inflight or awaiting receive */
 	atomic_t waiting;	/* number of requests awaiting receive */
-	atomic_t req_cnt;	/* number of requests */
+	atomic_t inactives;	/* number of inactive requests */
 	int sgat_elem_sz;	/* initialized to scatter_elem_sz */
 	int mmap_sz;		/* byte size of previous mmap() call */
 	unsigned long ffd_bm[1];	/* see SG_FFD_* defines above */
@@ -890,11 +892,13 @@  sg_rq_state_fail_msg(struct sg_fd *sfp, enum sg_rq_state exp_old_st,
 }
 #endif
 
+/* Functions ending in '_ulck' assume sfp->xa_lock held by caller. */
 static void
-sg_rq_state_force(struct sg_request *srp, enum sg_rq_state new_st)
+sg_rq_chg_state_force_ulck(struct sg_request *srp, enum sg_rq_state new_st)
 {
 	bool prev, want;
-	struct xarray *xafp = &srp->parentfp->srp_arr;
+	struct sg_fd *sfp = srp->parentfp;
+	struct xarray *xafp = &sfp->srp_arr;
 
 	atomic_set(&srp->rq_st, new_st);
 	want = (new_st == SG_RS_AWAIT_RCV);
@@ -908,15 +912,21 @@  sg_rq_state_force(struct sg_request *srp, enum sg_rq_state new_st)
 	want = (new_st == SG_RS_INACTIVE);
 	prev = xa_get_mark(xafp, srp->rq_idx, SG_XA_RQ_INACTIVE);
 	if (prev != want) {
-		if (want)
+		if (want) {
+			int prev_idx = READ_ONCE(sfp->low_used_idx);
+
+			if (prev_idx < 0 || srp->rq_idx < prev_idx ||
+			    !xa_get_mark(xafp, prev_idx, SG_XA_RQ_INACTIVE))
+				WRITE_ONCE(sfp->low_used_idx, srp->rq_idx);
 			__xa_set_mark(xafp, srp->rq_idx, SG_XA_RQ_INACTIVE);
-		else
+		} else {
 			__xa_clear_mark(xafp, srp->rq_idx, SG_XA_RQ_INACTIVE);
+		}
 	}
 }
 
 static void
-sg_rq_state_helper(struct xarray *xafp, struct sg_request *srp, int indic)
+sg_rq_chg_state_help(struct xarray *xafp, struct sg_request *srp, int indic)
 {
 	if (indic & 1)		/* from inactive state */
 		__xa_clear_mark(xafp, srp->rq_idx, SG_XA_RQ_INACTIVE);
@@ -945,45 +955,53 @@  static const int sg_rq_state_mul2arr[] = {2, 0, 8, 0};
  * spinlock is held.
  */
 static int
-sg_rq_state_chg(struct sg_request *srp, enum sg_rq_state old_st,
+sg_rq_chg_state(struct sg_request *srp, enum sg_rq_state old_st,
 		enum sg_rq_state new_st)
 {
 	enum sg_rq_state act_old_st;
-	int indic;
-	unsigned long iflags;
+	int indic = sg_rq_state_arr[(int)old_st] + sg_rq_state_mul2arr[(int)new_st];
 	struct sg_fd *sfp = srp->parentfp;
-	struct xarray *xafp = &sfp->srp_arr;
 
-	indic = sg_rq_state_arr[(int)old_st] +
-		sg_rq_state_mul2arr[(int)new_st];
-	act_old_st = (enum sg_rq_state)atomic_cmpxchg(&srp->rq_st, old_st,
-						      new_st);
-	if (act_old_st != old_st) {
-		SG_LOG(1, sfp, "%s: unexpected old state: %s\n", __func__,
-		       sg_rq_st_str(act_old_st, false));
-		return -EPROTOTYPE;	/* only used for this error type */
-	}
 	if (indic) {
+		unsigned long iflags;
+		struct xarray *xafp = &sfp->srp_arr;
+
 		xa_lock_irqsave(xafp, iflags);
-		if (new_st == SG_RS_INACTIVE)
-			WRITE_ONCE(sfp->prev_used_idx, srp->rq_idx);
-		sg_rq_state_helper(xafp, srp, indic);
+		act_old_st = (enum sg_rq_state)atomic_cmpxchg_relaxed(&srp->rq_st, old_st, new_st);
+		if (unlikely(act_old_st != old_st)) {
+			xa_unlock_irqrestore(xafp, iflags);
+			SG_LOG(1, sfp, "%s: unexpected old state: %s\n", __func__,
+			       sg_rq_st_str(act_old_st, false));
+			return -EPROTOTYPE;     /* only used for this error type */
+		}
+		if (new_st == SG_RS_INACTIVE) {
+			int prev_idx = READ_ONCE(sfp->low_used_idx);
+
+			if (prev_idx < 0 || srp->rq_idx < prev_idx ||
+			    !xa_get_mark(xafp, prev_idx, SG_XA_RQ_INACTIVE))
+				WRITE_ONCE(sfp->low_used_idx, srp->rq_idx);
+		}
+		sg_rq_chg_state_help(xafp, srp, indic);
 		xa_unlock_irqrestore(xafp, iflags);
+	} else {
+		act_old_st = (enum sg_rq_state)atomic_cmpxchg(&srp->rq_st, old_st, new_st);
+		if (unlikely(act_old_st != old_st)) {
+			SG_LOG(1, sfp, "%s: unexpected old state: %s\n", __func__,
+			       sg_rq_st_str(act_old_st, false));
+			return -EPROTOTYPE;     /* only used for this error type */
+		}
 	}
 	return 0;
 }
 
 static void
-sg_rq_state_chg_force(struct sg_request *srp, enum sg_rq_state new_st)
+sg_rq_chg_state_force(struct sg_request *srp, enum sg_rq_state new_st)
 {
 	unsigned long iflags;
-	struct sg_fd *sfp = srp->parentfp;
-	struct xarray *xafp = &sfp->srp_arr;
+	struct xarray *xafp = &srp->parentfp->srp_arr;
 
 	xa_lock_irqsave(xafp, iflags);
-	if (new_st == SG_RS_INACTIVE)
-		WRITE_ONCE(sfp->prev_used_idx, srp->rq_idx);
-	sg_rq_state_force(srp, new_st);
+	sg_rq_chg_state_force_ulck(srp, new_st);
 	xa_unlock_irqrestore(xafp, iflags);
 }
 
@@ -1008,11 +1026,13 @@  sg_execute_cmd(struct sg_fd *sfp, struct sg_request *srp)
 		at_head = !(srp->rq_flags & SG_FLAG_Q_AT_TAIL);
 
 	kref_get(&sfp->f_ref); /* sg_rq_end_io() does kref_put(). */
-	sg_rq_state_chg_force(srp, SG_RS_INFLIGHT);
+	sg_rq_chg_state_force(srp, SG_RS_INFLIGHT);
 
 	/* >>>>>>> send cmd/req off to other levels <<<<<<<< */
-	if (!sync)
+	if (!sync) {
 		atomic_inc(&sfp->submitted);
+		set_bit(SG_FRQ_COUNT_ACTIVE, srp->frq_bm);
+	}
 	blk_execute_rq_nowait(sdp->device->request_queue, sdp->disk,
 			      srp->rq, (int)at_head, sg_rq_end_io);
 }
@@ -1667,7 +1687,7 @@  sg_fill_request_element(struct sg_fd *sfp, struct sg_request *srp,
 static inline bool
 sg_rq_landed(struct sg_device *sdp, struct sg_request *srp)
 {
-	return atomic_read(&srp->rq_st) != SG_RS_INFLIGHT ||
+	return atomic_read_acquire(&srp->rq_st) != SG_RS_INFLIGHT ||
 	       unlikely(SG_IS_DETACHING(sdp));
 }
 
@@ -1683,6 +1703,8 @@  sg_wait_event_srp(struct file *filp, struct sg_fd *sfp, void __user *p,
 	enum sg_rq_state sr_st;
 	struct sg_device *sdp = sfp->parentdp;
 
+	if (atomic_read(&srp->rq_st) != SG_RS_INFLIGHT)
+		goto skip_wait;		/* and skip _acquire() */
 	SG_LOG(3, sfp, "%s: about to wait_event...()\n", __func__);
 	/* usually will be woken up by sg_rq_end_io() callback */
 	res = wait_event_interruptible(sfp->read_wait,
@@ -1695,14 +1717,16 @@  sg_wait_event_srp(struct file *filp, struct sg_fd *sfp, void __user *p,
 		       __func__, res);
 		return res;
 	}
+skip_wait:
 	if (unlikely(SG_IS_DETACHING(sdp))) {
-		atomic_set(&srp->rq_st, SG_RS_INACTIVE);
+		sg_rq_chg_state_force(srp, SG_RS_INACTIVE);
+		atomic_inc(&sfp->inactives);
 		return -ENODEV;
 	}
 	sr_st = atomic_read(&srp->rq_st);
 	if (unlikely(sr_st != SG_RS_AWAIT_RCV))
 		return -EPROTO;         /* Logic error */
-	res = sg_rq_state_chg(srp, sr_st, SG_RS_BUSY);
+	res = sg_rq_chg_state(srp, sr_st, SG_RS_BUSY);
 	if (unlikely(res)) {
 #if IS_ENABLED(SG_LOG_ACTIVE)
 		sg_rq_state_fail_msg(sfp, sr_st, SG_RS_BUSY, __func__);
@@ -1798,7 +1822,6 @@  sg_set_reserved_sz(struct sg_fd *sfp, int want_rsv_sz)
 	n_srp = sg_mk_srp_sgat(sfp, true /* can take time */, new_sz);
 	if (IS_ERR(n_srp))
 		return PTR_ERR(n_srp);
-	sg_rq_state_force(n_srp, SG_RS_INACTIVE);
 	/* new sg_request object, sized correctly is now available */
 try_again:
 	o_srp = sfp->rsv_srp;
@@ -1830,7 +1853,8 @@  sg_set_reserved_sz(struct sg_fd *sfp, int want_rsv_sz)
 		cxc_srp = __xa_cmpxchg(xafp, idx, o_srp, n_srp, GFP_ATOMIC);
 		if (o_srp == cxc_srp) {
 			sfp->rsv_srp = n_srp;
-			sg_rq_state_force(n_srp, SG_RS_INACTIVE);
+			sg_rq_chg_state_force_ulck(n_srp, SG_RS_INACTIVE);
+			/* don't bump inactives, since replaced an inactive */
 			xa_unlock_irqrestore(xafp, iflags);
 			SG_LOG(6, sfp, "%s: new rsv srp=0x%pK ++\n", __func__,
 			       n_srp);
@@ -2008,7 +2032,10 @@  sg_ioctl_common(struct file *filp, struct sg_device *sdp, struct sg_fd *sfp,
 		SG_LOG(3, sfp, "%s:    SG_GET_PACK_ID=%d\n", __func__, val);
 		return put_user(val, ip);
 	case SG_GET_NUM_WAITING:
-		return put_user(atomic_read(&sfp->waiting), ip);
+		val = atomic_read(&sfp->waiting);
+		if (val)
+			return put_user(val, ip);
+		return put_user(atomic_read_acquire(&sfp->waiting), ip);
 	case SG_GET_SG_TABLESIZE:
 		SG_LOG(3, sfp, "%s:    SG_GET_SG_TABLESIZE=%d\n", __func__,
 		       sdp->max_sgat_sz);
@@ -2221,11 +2248,16 @@  sg_compat_ioctl(struct file *filp, unsigned int cmd_in, unsigned long arg)
 static __poll_t
 sg_poll(struct file *filp, poll_table * wait)
 {
+	int num;
 	__poll_t p_res = 0;
 	struct sg_fd *sfp = filp->private_data;
 
-	poll_wait(filp, &sfp->read_wait, wait);
-	if (atomic_read(&sfp->waiting) > 0)
+	num = atomic_read(&sfp->waiting);
+	if (num < 1) {
+		poll_wait(filp, &sfp->read_wait, wait);
+		num = atomic_read(&sfp->waiting);
+	}
+	if (num > 0)
 		p_res = EPOLLIN | EPOLLRDNORM;
 
 	if (unlikely(SG_IS_DETACHING(sfp->parentdp)))
@@ -2474,32 +2506,13 @@  sg_rq_end_io(struct request *rq, blk_status_t status)
 	struct sg_device *sdp;
 	struct sg_fd *sfp;
 
-	if (!scsi_rp) {
-		WARN_ONCE("%s: scsi_req(rq) unexpectedly NULL\n", __func__);
-		return;
-	}
-	if (!srp) {
-		WARN_ONCE("%s: srp unexpectedly NULL\n", __func__);
-		return;
-	}
 	/* Expect 0 --> 1 transition, otherwise processed elsewhere */
 	if (unlikely(test_and_set_bit(SG_FRQ_BLK_PUT_REQ, srp->frq_bm))) {
 		pr_info("%s: srp=%pK already completed\n", __func__, srp);
 		return;
 	}
-	if (WARN_ON(atomic_read(&srp->rq_st) != SG_RS_INFLIGHT)) {
-		pr_warn("%s: bad rq_st=%d\n", __func__,
-			atomic_read(&srp->rq_st));
-		goto early_err;
-	}
 	sfp = srp->parentfp;
-	if (unlikely(!sfp)) {
-		WARN_ONCE(1, "%s: sfp unexpectedly NULL", __func__);
-		goto early_err;
-	}
 	sdp = sfp->parentdp;
-	if (unlikely(SG_IS_DETACHING(sdp)))
-		pr_info("%s: device detaching\n", __func__);
 
 	srp->rq_result = scsi_rp->result;
 	slen = min_t(int, scsi_rp->sense_len, SCSI_SENSE_BUFFERSIZE);
@@ -2552,10 +2565,20 @@  sg_rq_end_io(struct request *rq, blk_status_t status)
 			set_bit(SG_FRQ_DEACT_ORPHAN, srp->frq_bm);
 		}
 	}
-	if (!test_bit(SG_FRQ_SYNC_INVOC, srp->frq_bm))
-		atomic_inc(&sfp->waiting);
-	if (unlikely(sg_rq_state_chg(srp, SG_RS_INFLIGHT, rqq_state)))
-		pr_warn("%s: can't set rq_st\n", __func__);
+	if (test_bit(SG_FRQ_COUNT_ACTIVE, srp->frq_bm)) {
+		int num = atomic_inc_return(&sfp->waiting);
+
+		if (num < 2) {
+			WRITE_ONCE(sfp->low_await_idx, srp->rq_idx);
+		} else {
+			int l_await_idx = READ_ONCE(sfp->low_await_idx);
+
+			if (l_await_idx < 0 || srp->rq_idx < l_await_idx ||
+			    !xa_get_mark(&sfp->srp_arr, l_await_idx, SG_XA_RQ_AWAIT))
+				WRITE_ONCE(sfp->low_await_idx, srp->rq_idx);
+		}
+	}
+	sg_rq_chg_state_force(srp, rqq_state);
 	/*
 	 * Free the mid-level resources apart from the bio (if any). The bio's
 	 * blk_rq_unmap_user() can be called later from user context.
@@ -2574,12 +2597,6 @@  sg_rq_end_io(struct request *rq, blk_status_t status)
 		schedule_work(&srp->ew_orph.work);
 	}
 	return;
-
-early_err:
-	srp->rq = NULL;
-	if (scsi_rp)
-		scsi_req_free_cmd(scsi_rp);
-	blk_put_request(rq);
 }
 
 static const struct file_operations sg_fops = {
@@ -3068,7 +3085,7 @@  sg_finish_scsi_blk_rq(struct sg_request *srp)
 
 	SG_LOG(4, sfp, "%s: srp=0x%pK%s\n", __func__, srp,
 	       (srp->parentfp->rsv_srp == srp) ? " rsv" : "");
-	if (!test_bit(SG_FRQ_SYNC_INVOC, srp->frq_bm)) {
+	if (test_and_clear_bit(SG_FRQ_COUNT_ACTIVE, srp->frq_bm)) {
 		atomic_dec(&sfp->submitted);
 		atomic_dec(&sfp->waiting);
 	}
@@ -3264,19 +3281,28 @@  sg_find_srp_by_id(struct sg_fd *sfp, int pack_id)
 	bool second = false;
 	int res;
 	int num_waiting = atomic_read(&sfp->waiting);
-	unsigned long idx, start_idx, end_idx;
+	int l_await_idx = READ_ONCE(sfp->low_await_idx);
+	unsigned long idx, s_idx;
+	unsigned long end_idx = ULONG_MAX;
 	struct sg_request *srp = NULL;
 	struct xarray *xafp = &sfp->srp_arr;
 
-	if (num_waiting < 1)
-		return NULL;
+	if (num_waiting < 1) {
+		num_waiting = atomic_read_acquire(&sfp->waiting);
+		if (num_waiting < 1)
+			return NULL;
+	}
+
+	s_idx = (l_await_idx < 0) ? 0 : l_await_idx;
+	idx = s_idx;
 	if (unlikely(search_for_1)) {
-		xa_for_each_marked(xafp, idx, srp, SG_XA_RQ_AWAIT) {
-			if (!srp)
-				continue;
+second_time:
+		for (srp = xa_find(xafp, &idx, end_idx, SG_XA_RQ_AWAIT);
+		     srp;
+		     srp = xa_find_after(xafp, &idx, end_idx, SG_XA_RQ_AWAIT)) {
 			if (srp->pack_id != pack_id)
 				continue;
-			res = sg_rq_state_chg(srp, SG_RS_AWAIT_RCV, SG_RS_BUSY);
+			res = sg_rq_chg_state(srp, SG_RS_AWAIT_RCV, SG_RS_BUSY);
 			if (likely(res == 0))
 				goto good;
 			/* else another caller got it, move on */
@@ -3286,6 +3312,14 @@  sg_find_srp_by_id(struct sg_fd *sfp, int pack_id)
 			}
 			break;
 		}
+		/* If not found so far, need to wrap around and search [0 ... s_idx) */
+		if (!srp && !second && s_idx > 0) {
+			end_idx = s_idx - 1;
+			s_idx = 0;
+			idx = s_idx;
+			second = true;
+			goto second_time;
+		}
 	} else {
 		/*
 		 * Searching for _any_ request is the more likely usage. Start searching with the
@@ -3294,27 +3328,25 @@  sg_find_srp_by_id(struct sg_fd *sfp, int pack_id)
 		 * is ready. If there is no queuing and the "last used" has been re-used then the
 		 * first (relative) position will be the request we want.
 		 */
-		start_idx = READ_ONCE(sfp->prev_used_idx);
-		end_idx = ULONG_MAX;
-second_time:
-		idx = start_idx;
+second_time2:
 		for (srp = xa_find(xafp, &idx, end_idx, SG_XA_RQ_AWAIT);
 		     srp;
 		     srp = xa_find_after(xafp, &idx, end_idx, SG_XA_RQ_AWAIT)) {
-			res = sg_rq_state_chg(srp, SG_RS_AWAIT_RCV, SG_RS_BUSY);
-			if (likely(res == 0))
+			res = sg_rq_chg_state(srp, SG_RS_AWAIT_RCV, SG_RS_BUSY);
+			if (likely(res == 0)) {
+				WRITE_ONCE(sfp->low_await_idx, idx + 1);
 				goto good;
+			}
 #if IS_ENABLED(SG_LOG_ACTIVE)
-			else
-				sg_rq_state_fail_msg(sfp, SG_RS_AWAIT_RCV, SG_RS_BUSY, __func__);
+			sg_rq_state_fail_msg(sfp, SG_RS_AWAIT_RCV, SG_RS_BUSY, __func__);
 #endif
 		}
-		/* If not found so far, need to wrap around and search [0 ... start_idx) */
-		if (!srp && !second && start_idx > 0) {
-			end_idx = start_idx - 1;
-			start_idx = 0;
+		if (!srp && !second && s_idx > 0) {
+			end_idx = s_idx - 1;
+			s_idx = 0;
+			idx = s_idx;
 			second = true;
-			goto second_time;
+			goto second_time2;
 		}
 	}
 	/* here if one of above loops does _not_ find a match */
@@ -3433,11 +3465,12 @@  sg_setup_req(struct sg_comm_wr_t *cwrp, int dxfr_len)
 	bool mk_new_srp = true;
 	bool try_harder = false;
 	bool second = false;
-	int num_inactive = 0;
-	unsigned long idx, start_idx, end_idx, iflags;
+	bool has_inactive = false;
+	int l_used_idx;
+	unsigned long idx, s_idx, end_idx, iflags;
 	struct sg_fd *fp = cwrp->sfp;
 	struct sg_request *r_srp = NULL;	/* request to return */
-	struct sg_request *last_srp = NULL;
+	struct sg_request *low_srp = NULL;
 	struct xarray *xafp = &fp->srp_arr;
 	__maybe_unused const char *cp;
 
@@ -3446,49 +3479,70 @@  sg_setup_req(struct sg_comm_wr_t *cwrp, int dxfr_len)
 	if (xa_empty(xafp)) {
 		act_empty = true;
 		mk_new_srp = true;
+	} else if (atomic_read(&fp->inactives) <= 0) {
+		mk_new_srp = true;
 	} else if (!try_harder && dxfr_len < SG_DEF_SECTOR_SZ) {
+		l_used_idx = READ_ONCE(fp->low_used_idx);
+		s_idx = (l_used_idx < 0) ? 0 : l_used_idx;
+		if (l_used_idx >= 0 && xa_get_mark(xafp, s_idx, SG_XA_RQ_INACTIVE)) {
+			r_srp = xa_load(xafp, s_idx);
+			if (r_srp && r_srp->sgat_h.buflen <= SG_DEF_SECTOR_SZ) {
+				if (sg_rq_chg_state(r_srp, SG_RS_INACTIVE, SG_RS_BUSY) == 0) {
+					found = true;
+					atomic_dec(&fp->inactives);
+					goto have_existing;
+				}
+			}
+		}
 		xa_for_each_marked(xafp, idx, r_srp, SG_XA_RQ_INACTIVE) {
-			++num_inactive;
-			if (dxfr_len < SG_DEF_SECTOR_SZ)
-				last_srp = r_srp;
+			has_inactive = true;
+			if (!low_srp && dxfr_len < SG_DEF_SECTOR_SZ) {
+				low_srp = r_srp;
+				break;
+			}
 		}
-		/* If dxfr_len is small, use last inactive request */
-		if (last_srp) {
-			r_srp = last_srp;
-			if (sg_rq_state_chg(r_srp, SG_RS_INACTIVE, SG_RS_BUSY))
+		/* If dxfr_len is small, use lowest inactive request */
+		if (low_srp) {
+			r_srp = low_srp;
+			if (sg_rq_chg_state(r_srp, SG_RS_INACTIVE, SG_RS_BUSY))
 				goto start_again; /* gone to another thread */
+			atomic_dec(&fp->inactives);
 			cp = "toward end of srp_arr";
 			found = true;
 		}
 	} else {
-		start_idx = READ_ONCE(fp->prev_used_idx);
+		l_used_idx = READ_ONCE(fp->low_used_idx);
+		s_idx = (l_used_idx < 0) ? 0 : l_used_idx;
+		idx = s_idx;
 		end_idx = ULONG_MAX;
 second_time:
-		idx = start_idx;
 		for (r_srp = xa_find(xafp, &idx, end_idx, SG_XA_RQ_INACTIVE);
 		     r_srp;
 		     r_srp = xa_find_after(xafp, &idx, end_idx, SG_XA_RQ_INACTIVE)) {
 			if (r_srp->sgat_h.buflen >= dxfr_len) {
-				if (sg_rq_state_chg(r_srp, SG_RS_INACTIVE, SG_RS_BUSY))
+				if (sg_rq_chg_state(r_srp, SG_RS_INACTIVE, SG_RS_BUSY))
 					continue;
+				atomic_dec(&fp->inactives);
+				WRITE_ONCE(fp->low_used_idx, idx + 1);
 				cp = "near front of srp_arr";
 				found = true;
 				break;
 			}
 		}
 		/* If not found so far, need to wrap around and search [0 ... start_idx) */
-		if (!r_srp && !second && start_idx > 0) {
-			end_idx = start_idx - 1;
-			start_idx = 0;
+		if (!r_srp && !second && s_idx > 0) {
+			end_idx = s_idx - 1;
+			s_idx = 0;
+			idx = s_idx;
 			second = true;
 			goto second_time;
 		}
 	}
+have_existing:
 	if (found) {
 		r_srp->in_resid = 0;
 		r_srp->rq_info = 0;
 		r_srp->sense_len = 0;
-		WRITE_ONCE(fp->prev_used_idx, r_srp->rq_idx);
 		mk_new_srp = false;
 	} else {
 		mk_new_srp = true;
@@ -3497,7 +3551,6 @@  sg_setup_req(struct sg_comm_wr_t *cwrp, int dxfr_len)
 		bool allow_cmd_q = test_bit(SG_FFD_CMD_Q, fp->ffd_bm);
 		int res;
 		u32 n_idx;
-		struct xa_limit xal = { .max = 0, .min = 0 };
 
 		cp = "new";
 		if (!allow_cmd_q && atomic_read(&fp->submitted) > 0) {
@@ -3509,16 +3562,14 @@  sg_setup_req(struct sg_comm_wr_t *cwrp, int dxfr_len)
 		r_srp = sg_mk_srp_sgat(fp, act_empty, dxfr_len);
 		if (IS_ERR(r_srp)) {
 			if (!try_harder && dxfr_len < SG_DEF_SECTOR_SZ &&
-			    num_inactive > 0) {
+			    has_inactive) {
 				try_harder = true;
 				goto start_again;
 			}
 			goto fini;
 		}
-		atomic_set(&r_srp->rq_st, SG_RS_BUSY);
 		xa_lock_irqsave(xafp, iflags);
-		xal.max = atomic_inc_return(&fp->req_cnt);
-		res = __xa_alloc(xafp, &n_idx, r_srp, xal, GFP_KERNEL);
+		res = __xa_alloc(xafp, &n_idx, r_srp, xa_limit_32b, GFP_KERNEL);
 		xa_unlock_irqrestore(xafp, iflags);
 		if (res < 0) {
 			SG_LOG(1, fp, "%s: xa_alloc() failed, errno=%d\n",
@@ -3561,7 +3612,8 @@  sg_deact_request(struct sg_fd *sfp, struct sg_request *srp)
 	sbp = srp->sense_bp;
 	srp->sense_bp = NULL;
 	WRITE_ONCE(srp->frq_bm[0], 0);
-	sg_rq_state_chg_force(srp, SG_RS_INACTIVE);
+	sg_rq_chg_state_force(srp, SG_RS_INACTIVE);
+	atomic_inc(&sfp->inactives);
 	/* maybe orphaned req, thus never read */
 	if (sbp)
 		mempool_free(sbp, sg_sense_pool);
@@ -3580,7 +3632,6 @@  sg_add_sfp(struct sg_device *sdp)
 	struct sg_request *srp = NULL;
 	struct xarray *xadp = &sdp->sfp_arr;
 	struct xarray *xafp;
-	struct xa_limit xal;
 
 	sfp = kzalloc(sizeof(*sfp), GFP_ATOMIC | __GFP_NOWARN);
 	if (!sfp)
@@ -3597,8 +3648,6 @@  sg_add_sfp(struct sg_device *sdp)
 	__assign_bit(SG_FFD_CMD_Q, sfp->ffd_bm, SG_DEF_COMMAND_Q);
 	__assign_bit(SG_FFD_KEEP_ORPHAN, sfp->ffd_bm, SG_DEF_KEEP_ORPHAN);
 	__assign_bit(SG_FFD_Q_AT_TAIL, sfp->ffd_bm, SG_DEFAULT_Q_AT);
-	atomic_set(&sfp->submitted, 0);
-	atomic_set(&sfp->waiting, 0);
 	/*
 	 * SG_SCATTER_SZ initializes scatter_elem_sz but different value may
 	 * be given as driver/module parameter (e.g. 'scatter_elem_sz=8192').
@@ -3610,7 +3659,7 @@  sg_add_sfp(struct sg_device *sdp)
 	sfp->parentdp = sdp;
 	atomic_set(&sfp->submitted, 0);
 	atomic_set(&sfp->waiting, 0);
-	atomic_set(&sfp->req_cnt, 0);
+	atomic_set(&sfp->inactives, 0);
 
 	if (unlikely(SG_IS_DETACHING(sdp))) {
 		SG_LOG(1, sfp, "%s: detaching\n", __func__);
@@ -3622,8 +3671,6 @@  sg_add_sfp(struct sg_device *sdp)
 
 	rbuf_len = min_t(int, sg_big_buff, sdp->max_sgat_sz);
 	if (rbuf_len > 0) {
-		struct xa_limit xalrq = { .max = 0, .min = 0 };
-
 		srp = sg_build_reserve(sfp, rbuf_len);
 		if (IS_ERR(srp)) {
 			kfree(sfp);
@@ -3639,8 +3686,7 @@  sg_add_sfp(struct sg_device *sdp)
 			       __func__, rbuf_len, srp->sgat_h.buflen);
 		}
 		xa_lock_irqsave(xafp, iflags);
-		xalrq.max = atomic_inc_return(&sfp->req_cnt);
-		res = __xa_alloc(xafp, &idx, srp, xalrq, GFP_ATOMIC);
+		res = __xa_alloc(xafp, &idx, srp, xa_limit_32b, GFP_ATOMIC);
 		xa_unlock_irqrestore(xafp, iflags);
 		if (res < 0) {
 			SG_LOG(1, sfp, "%s: xa_alloc(srp) bad, errno=%d\n",
@@ -3652,20 +3698,19 @@  sg_add_sfp(struct sg_device *sdp)
 		}
 		srp->rq_idx = idx;
 		srp->parentfp = sfp;
-		sg_rq_state_chg_force(srp, SG_RS_INACTIVE);
+		sg_rq_chg_state_force(srp, SG_RS_INACTIVE);
+		atomic_inc(&sfp->inactives);
 	}
 	if (!reduced) {
 		SG_LOG(4, sfp, "%s: built reserve buflen=%d\n", __func__,
 		       rbuf_len);
 	}
 	xa_lock_irqsave(xadp, iflags);
-	xal.min = 0;
-	xal.max = atomic_read(&sdp->open_cnt);
-	res = __xa_alloc(xadp, &idx, sfp, xal, GFP_KERNEL);
+	res = __xa_alloc(xadp, &idx, sfp, xa_limit_32b, GFP_KERNEL);
 	xa_unlock_irqrestore(xadp, iflags);
 	if (res < 0) {
 		pr_warn("%s: xa_alloc(sdp) bad, o_count=%d, errno=%d\n",
-			__func__, xal.max, -res);
+			__func__, atomic_read(&sdp->open_cnt), -res);
 		if (srp) {
 			sg_remove_sgat(srp);
 			kfree(srp);
@@ -4079,12 +4124,13 @@  sg_proc_debug_fd(struct sg_fd *fp, char *obp, int len, unsigned long idx)
 		       (int)test_bit(SG_FFD_FORCE_PACKID, fp->ffd_bm),
 		       (int)test_bit(SG_FFD_KEEP_ORPHAN, fp->ffd_bm),
 		       fp->ffd_bm[0]);
-	n += scnprintf(obp + n, len - n, "   mmap_sz=%d prev_used_idx=%d\n",
-		       fp->mmap_sz, fp->prev_used_idx);
 	n += scnprintf(obp + n, len - n,
-		       "   submitted=%d waiting=%d   open thr_id=%d\n",
+		       "   mmap_sz=%d low_used_idx=%d low_await_idx=%d\n",
+		       fp->mmap_sz, READ_ONCE(fp->low_used_idx), READ_ONCE(fp->low_await_idx));
+	n += scnprintf(obp + n, len - n,
+		       "   submitted=%d waiting=%d inactives=%d   open thr_id=%d\n",
 		       atomic_read(&fp->submitted),
-		       atomic_read(&fp->waiting), fp->tid);
+		       atomic_read(&fp->waiting), atomic_read(&fp->inactives), fp->tid);
 	k = 0;
 	xa_lock_irqsave(&fp->srp_arr, iflags);
 	xa_for_each(&fp->srp_arr, idx, srp) {