| Message ID | 20250325134000.575794-1-ptesarik@suse.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] usb: core: warn if a GFP zone flag is passed to hcd_buffer_alloc() | expand |
On Tue, 25 Mar 2025 14:40:00 +0100 Petr Tesarik <ptesarik@suse.com> wrote: > Remove a misleading comment and issue a warning if a zone modifier is > specified when allocating a hcd buffer. > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > - PIO mode can use any kernel-addressable memory > - dma_alloc_coherent() ignores memory zone bits > > This function is called by usb_alloc_coherent() and indirectly by > usb_submit_urb(). Despite the comment, no in-tree users currently pass > GFP_DMA. Let me provide a bit of background on this patch. My actual goal is to remove the DMA zone; I'm now going through core code and removing GFP_DMA references that somehow look incorrect to me. I hope this preparation makes it easier to review the removal of GFP_DMA later. Petr T > Signed-off-by: Petr Tesarik <ptesarik@suse.com> > --- > drivers/usb/core/buffer.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c > index 87230869e1fa..10844cd42e66 100644 > --- a/drivers/usb/core/buffer.c > +++ b/drivers/usb/core/buffer.c > @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd) > } > > > -/* sometimes alloc/free could use kmalloc with GFP_DMA, for > - * better sharing and to leverage mm/slab.c intelligence. > - */ > - > void *hcd_buffer_alloc( > struct usb_bus *bus, > size_t size, > @@ -128,6 +124,12 @@ void *hcd_buffer_alloc( > if (hcd->localmem_pool) > return gen_pool_dma_alloc(hcd->localmem_pool, size, dma); > > + /* > + * Zone modifiers are ignored by DMA API, and PIO should always use > + * GFP_KERNEL. > + */ > + WARN_ON_ONCE(mem_flags & GFP_ZONEMASK); > + > /* some USB hosts just use PIO */ > if (!hcd_uses_dma(hcd)) { > *dma = ~(dma_addr_t) 0;
On Tue, 25 Mar 2025 14:40:00 +0100 Petr Tesarik <ptesarik@suse.com> wrote: > Remove a misleading comment and issue a warning if a zone modifier is > specified when allocating a hcd buffer. > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > - PIO mode can use any kernel-addressable memory > - dma_alloc_coherent() ignores memory zone bits > > This function is called by usb_alloc_coherent() and indirectly by > usb_submit_urb(). Despite the comment, no in-tree users currently pass > GFP_DMA. > > Signed-off-by: Petr Tesarik <ptesarik@suse.com> I know this was posted during the merge window, but that's now over. Any comment on this patch? Petr T
On Wed, Apr 09, 2025 at 05:40:36PM +0200, Petr Tesařík wrote: > On Tue, 25 Mar 2025 14:40:00 +0100 > Petr Tesarik <ptesarik@suse.com> wrote: > > > Remove a misleading comment and issue a warning if a zone modifier is > > specified when allocating a hcd buffer. > > > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > > - PIO mode can use any kernel-addressable memory > > - dma_alloc_coherent() ignores memory zone bits > > > > This function is called by usb_alloc_coherent() and indirectly by > > usb_submit_urb(). Despite the comment, no in-tree users currently pass > > GFP_DMA. > > > > Signed-off-by: Petr Tesarik <ptesarik@suse.com> > > I know this was posted during the merge window, but that's now over. > Any comment on this patch? Please give me a chance to catch up, it's been non-stop conference travel during the merge window, and through this week :( thanks, greg k-h
On Tue, Mar 25, 2025 at 02:40:00PM +0100, Petr Tesarik wrote: > Remove a misleading comment and issue a warning if a zone modifier is > specified when allocating a hcd buffer. > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > - PIO mode can use any kernel-addressable memory > - dma_alloc_coherent() ignores memory zone bits > > This function is called by usb_alloc_coherent() and indirectly by > usb_submit_urb(). Despite the comment, no in-tree users currently pass > GFP_DMA. > > Signed-off-by: Petr Tesarik <ptesarik@suse.com> > --- > drivers/usb/core/buffer.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c > index 87230869e1fa..10844cd42e66 100644 > --- a/drivers/usb/core/buffer.c > +++ b/drivers/usb/core/buffer.c > @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd) > } > > > -/* sometimes alloc/free could use kmalloc with GFP_DMA, for > - * better sharing and to leverage mm/slab.c intelligence. > - */ > - > void *hcd_buffer_alloc( > struct usb_bus *bus, > size_t size, > @@ -128,6 +124,12 @@ void *hcd_buffer_alloc( > if (hcd->localmem_pool) > return gen_pool_dma_alloc(hcd->localmem_pool, size, dma); > > + /* > + * Zone modifiers are ignored by DMA API, and PIO should always use > + * GFP_KERNEL. > + */ > + WARN_ON_ONCE(mem_flags & GFP_ZONEMASK); You just rebooted the box if this happens, do you REALLY want to do that? People generally do not like their data lost :( Why not just fix the callers, OR if this really isn't going to be allowed, return an error and just fail the whole submission? And stick around to fix up all of the drivers that end up triggering this... thanks, greg k-h
On Fri, 11 Apr 2025 15:57:19 +0200 Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote: > On Tue, Mar 25, 2025 at 02:40:00PM +0100, Petr Tesarik wrote: > > Remove a misleading comment and issue a warning if a zone modifier is > > specified when allocating a hcd buffer. > > > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > > - PIO mode can use any kernel-addressable memory > > - dma_alloc_coherent() ignores memory zone bits > > > > This function is called by usb_alloc_coherent() and indirectly by > > usb_submit_urb(). Despite the comment, no in-tree users currently pass > > GFP_DMA. > > > > Signed-off-by: Petr Tesarik <ptesarik@suse.com> > > --- > > drivers/usb/core/buffer.c | 10 ++++++---- > > 1 file changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c > > index 87230869e1fa..10844cd42e66 100644 > > --- a/drivers/usb/core/buffer.c > > +++ b/drivers/usb/core/buffer.c > > @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd) > > } > > > > > > -/* sometimes alloc/free could use kmalloc with GFP_DMA, for > > - * better sharing and to leverage mm/slab.c intelligence. > > - */ > > - > > void *hcd_buffer_alloc( > > struct usb_bus *bus, > > size_t size, > > @@ -128,6 +124,12 @@ void *hcd_buffer_alloc( > > if (hcd->localmem_pool) > > return gen_pool_dma_alloc(hcd->localmem_pool, size, dma); > > > > + /* > > + * Zone modifiers are ignored by DMA API, and PIO should always use > > + * GFP_KERNEL. > > + */ > > + WARN_ON_ONCE(mem_flags & GFP_ZONEMASK); > > You just rebooted the box if this happens, do you REALLY want to do > that? People generally do not like their data lost :( FWIW my box does not reboot on a warning. But I admit there are people who want to run their systems with panic_on_warn (although I suspect they already experience some sudden reboots, so they had better be prepared). > Why not just fix the callers, OR if this really isn't going to be > allowed, return an error and just fail the whole submission? And stick > around to fix up all of the drivers that end up triggering this... That's the point. AFAICS there are _no_ in-tree callers that would pass GFP_DMA or GFP_DMA32 to hcd_buffer_alloc(), directly or indirectly. But nobody should be tempted to add the flag, because I cannot imagine how that would ever be the right thing to do. I can change it back to mem_flags &= ~GFP_ZONEMASK to fix it silently; I simply thought that driver authors may appreciate a warning that they're trying to do something silly. Whatever works for you, but please keep in mind that there seems to be agreement among mm people that DMA and DMA32 zones should be removed from the kernel eventually. Petr T
On Mon, 14 Apr 2025 09:12:09 +0200 Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote: > On Mon, Apr 14, 2025 at 09:02:16AM +0200, Petr Tesarik wrote: > > On Fri, 11 Apr 2025 15:57:19 +0200 > > Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote: > > > > > On Tue, Mar 25, 2025 at 02:40:00PM +0100, Petr Tesarik wrote: > > > > Remove a misleading comment and issue a warning if a zone modifier is > > > > specified when allocating a hcd buffer. > > > > > > > > There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): > > > > - PIO mode can use any kernel-addressable memory > > > > - dma_alloc_coherent() ignores memory zone bits > > > > > > > > This function is called by usb_alloc_coherent() and indirectly by > > > > usb_submit_urb(). Despite the comment, no in-tree users currently pass > > > > GFP_DMA. > > > > > > > > Signed-off-by: Petr Tesarik <ptesarik@suse.com> > > > > --- > > > > drivers/usb/core/buffer.c | 10 ++++++---- > > > > 1 file changed, 6 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c > > > > index 87230869e1fa..10844cd42e66 100644 > > > > --- a/drivers/usb/core/buffer.c > > > > +++ b/drivers/usb/core/buffer.c > > > > @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd) > > > > } > > > > > > > > > > > > -/* sometimes alloc/free could use kmalloc with GFP_DMA, for > > > > - * better sharing and to leverage mm/slab.c intelligence. > > > > - */ > > > > - > > > > void *hcd_buffer_alloc( > > > > struct usb_bus *bus, > > > > size_t size, > > > > @@ -128,6 +124,12 @@ void *hcd_buffer_alloc( > > > > if (hcd->localmem_pool) > > > > return gen_pool_dma_alloc(hcd->localmem_pool, size, dma); > > > > > > > > + /* > > > > + * Zone modifiers are ignored by DMA API, and PIO should always use > > > > + * GFP_KERNEL. > > > > + */ > > > > + WARN_ON_ONCE(mem_flags & GFP_ZONEMASK); > > > > > > You just rebooted the box if this happens, do you REALLY want to do > > > that? People generally do not like their data lost :( > > > > FWIW my box does not reboot on a warning. But I admit there are people > > who want to run their systems with panic_on_warn (although I suspect > > they already experience some sudden reboots, so they had better be > > prepared). > > There are billions of Linux systems out there with panic-on-warn enabled :( > > > > Why not just fix the callers, OR if this really isn't going to be > > > allowed, return an error and just fail the whole submission? And stick > > > around to fix up all of the drivers that end up triggering this... > > > > That's the point. AFAICS there are _no_ in-tree callers that would pass > > GFP_DMA or GFP_DMA32 to hcd_buffer_alloc(), directly or indirectly. But > > nobody should be tempted to add the flag, because I cannot imagine how > > that would ever be the right thing to do. > > > > I can change it back to mem_flags &= ~GFP_ZONEMASK to fix it silently; > > I simply thought that driver authors may appreciate a warning that > > they're trying to do something silly. > > A warning is fine, but don't reboot a box please. dev_warn() with a > ratelimit and then return an error perhaps? If we're concerned about breaking existing systems in the wild, then we should merely issue a warning that the flag is ignored. So, probably a ratelimited dev_warn() and continue operation. > > Whatever works for you, but please keep in mind that there seems to be > > agreement among mm people that DMA and DMA32 zones should be removed > > from the kernel eventually. > > I agree, they should be removed as they don't do what people think they > do. So why not just remove them entirely, otherwise are you going to go > and add this type of checking to all bus subsystems? I'm kind of testing grounds here. But yes, I'm browsing all in-tree occurrences of GFP_DMA and GFP_DMA32, looking for corner cases that may break. So far, I have found exactly one user of the DMA zone who appears to be quite right: s390 I/O channels to cope with the legacy 31-bit addressing of the CCW instruction. JFYI. Petr T
On 16.04.25 12:47, Petr Tesarik wrote: > If I stay with the USB buffer allocations, AFAICS the mem_flags > parameter should be used only for non-zone flags. If you specify, > GFP_DMA here, it will have no impact whatsoever on allocating DMA > buffers. It may unnecessarily allocate from the DMA zone for doing PIO. Yes. But we should not limit enforcement of such a _new_ policy to one method in order to fix a hypothetical issue. There is just no need for action. > Now I think I should really write an article for LWN to debunk some > myths about GFP_DMA. Well, if you go to that trouble an explanation of why memflags are passed in USB at all and how DMA works in general would be productive. Regards Oliver
diff --git a/drivers/usb/core/buffer.c b/drivers/usb/core/buffer.c index 87230869e1fa..10844cd42e66 100644 --- a/drivers/usb/core/buffer.c +++ b/drivers/usb/core/buffer.c @@ -108,10 +108,6 @@ void hcd_buffer_destroy(struct usb_hcd *hcd) } -/* sometimes alloc/free could use kmalloc with GFP_DMA, for - * better sharing and to leverage mm/slab.c intelligence. - */ - void *hcd_buffer_alloc( struct usb_bus *bus, size_t size, @@ -128,6 +124,12 @@ void *hcd_buffer_alloc( if (hcd->localmem_pool) return gen_pool_dma_alloc(hcd->localmem_pool, size, dma); + /* + * Zone modifiers are ignored by DMA API, and PIO should always use + * GFP_KERNEL. + */ + WARN_ON_ONCE(mem_flags & GFP_ZONEMASK); + /* some USB hosts just use PIO */ if (!hcd_uses_dma(hcd)) { *dma = ~(dma_addr_t) 0;
Remove a misleading comment and issue a warning if a zone modifier is specified when allocating a hcd buffer. There is no valid use case for a GFP zone modifier in hcd_buffer_alloc(): - PIO mode can use any kernel-addressable memory - dma_alloc_coherent() ignores memory zone bits This function is called by usb_alloc_coherent() and indirectly by usb_submit_urb(). Despite the comment, no in-tree users currently pass GFP_DMA. Signed-off-by: Petr Tesarik <ptesarik@suse.com> --- drivers/usb/core/buffer.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)