diff mbox series

[v2,1/6] mm: introduce vma->vm_flags modifier functions

Message ID 20230125083851.27759-2-surenb@google.com
State New
Headers show
Series introduce vm_flags modifier functions | expand

Commit Message

Suren Baghdasaryan Jan. 25, 2023, 8:38 a.m. UTC
vm_flags are among VMA attributes which affect decisions like VMA merging
and splitting. Therefore all vm_flags modifications are performed after
taking exclusive mmap_lock to prevent vm_flags updates racing with such
operations. Introduce modifier functions for vm_flags to be used whenever
flags are updated. This way we can better check and control correct
locking behavior during these updates.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h       | 37 +++++++++++++++++++++++++++++++++++++
 include/linux/mm_types.h |  8 +++++++-
 2 files changed, 44 insertions(+), 1 deletion(-)

Comments

Matthew Wilcox Jan. 25, 2023, 6:33 p.m. UTC | #1
On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> +/* Use when VMA is not part of the VMA tree and needs no locking */
> +static inline void init_vm_flags(struct vm_area_struct *vma,
> +				 unsigned long flags)
> +{
> +	vma->vm_flags = flags;

vm_flags are supposed to have type vm_flags_t.  That's not been
fully realised yet, but perhaps we could avoid making it worse?

>  	pgprot_t vm_page_prot;
> -	unsigned long vm_flags;		/* Flags, see mm.h. */
> +
> +	/*
> +	 * Flags, see mm.h.
> +	 * WARNING! Do not modify directly.
> +	 * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> +	 */
> +	unsigned long vm_flags;

Including changing this line to vm_flags_t
Suren Baghdasaryan Jan. 25, 2023, 7:21 p.m. UTC | #2
On Wed, Jan 25, 2023 at 10:37 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jan 25, 2023 at 08:49:50AM -0800, Suren Baghdasaryan wrote:
> > On Wed, Jan 25, 2023 at 1:10 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > +     /*
> > > > +      * Flags, see mm.h.
> > > > +      * WARNING! Do not modify directly.
> > > > +      * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> > > > +      */
> > > > +     unsigned long vm_flags;
> > >
> > > We have __private and ACCESS_PRIVATE() to help with enforcing this.
> >
> > Thanks for pointing this out, Peter! I guess for that I'll need to
> > convert all read accesses and provide get_vm_flags() too? That will
> > cause some additional churt (a quick search shows 801 hits over 248
> > files) but maybe it's worth it? I think Michal suggested that too in
> > another patch. Should I do that while we are at it?
>
> Here's a trick I saw somewhere in the VFS:
>
>         union {
>                 const vm_flags_t vm_flags;
>                 vm_flags_t __private __vm_flags;
>         };
>
> Now it can be read by anybody but written only by those using
> ACCESS_PRIVATE.

Huh, this is quite nice! I think it does not save us from the cases
when vma->vm_flags is passed by a reference and modified indirectly,
like in ksm_madvise()? Though maybe such usecases are so rare (I found
only 2 cases) that we can ignore this?
Mike Rapoport Jan. 26, 2023, 9:17 a.m. UTC | #3
On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> vm_flags are among VMA attributes which affect decisions like VMA merging
> and splitting. Therefore all vm_flags modifications are performed after
> taking exclusive mmap_lock to prevent vm_flags updates racing with such
> operations. Introduce modifier functions for vm_flags to be used whenever
> flags are updated. This way we can better check and control correct
> locking behavior during these updates.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm.h       | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/mm_types.h |  8 +++++++-
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c2f62bdce134..b71f2809caac 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -627,6 +627,43 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
>  	INIT_LIST_HEAD(&vma->anon_vma_chain);
>  }
>  
> +/* Use when VMA is not part of the VMA tree and needs no locking */
> +static inline void init_vm_flags(struct vm_area_struct *vma,
> +				 unsigned long flags)

I'd suggest to make it vm_flags_init() etc.
Except that

Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>

> +{
> +	vma->vm_flags = flags;
> +}
> +
> +/* Use when VMA is part of the VMA tree and modifications need coordination */
> +static inline void reset_vm_flags(struct vm_area_struct *vma,
> +				  unsigned long flags)
> +{
> +	mmap_assert_write_locked(vma->vm_mm);
> +	init_vm_flags(vma, flags);
> +}
> +
> +static inline void set_vm_flags(struct vm_area_struct *vma,
> +				unsigned long flags)
> +{
> +	mmap_assert_write_locked(vma->vm_mm);
> +	vma->vm_flags |= flags;
> +}
> +
> +static inline void clear_vm_flags(struct vm_area_struct *vma,
> +				  unsigned long flags)
> +{
> +	mmap_assert_write_locked(vma->vm_mm);
> +	vma->vm_flags &= ~flags;
> +}
> +
> +static inline void mod_vm_flags(struct vm_area_struct *vma,
> +				unsigned long set, unsigned long clear)
> +{
> +	mmap_assert_write_locked(vma->vm_mm);
> +	vma->vm_flags |= set;
> +	vma->vm_flags &= ~clear;
> +}
> +
>  static inline void vma_set_anonymous(struct vm_area_struct *vma)
>  {
>  	vma->vm_ops = NULL;
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 2d6d790d9bed..6c7c70bf50dd 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -491,7 +491,13 @@ struct vm_area_struct {
>  	 * See vmf_insert_mixed_prot() for discussion.
>  	 */
>  	pgprot_t vm_page_prot;
> -	unsigned long vm_flags;		/* Flags, see mm.h. */
> +
> +	/*
> +	 * Flags, see mm.h.
> +	 * WARNING! Do not modify directly.
> +	 * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
> +	 */
> +	unsigned long vm_flags;
>  
>  	/*
>  	 * For areas with an address space and backing store,
> -- 
> 2.39.1
> 
>
Mike Rapoport Jan. 26, 2023, 2:50 p.m. UTC | #4
On Thu, Jan 26, 2023 at 11:17:09AM +0200, Mike Rapoport wrote:
> On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> > vm_flags are among VMA attributes which affect decisions like VMA merging
> > and splitting. Therefore all vm_flags modifications are performed after
> > taking exclusive mmap_lock to prevent vm_flags updates racing with such
> > operations. Introduce modifier functions for vm_flags to be used whenever
> > flags are updated. This way we can better check and control correct
> > locking behavior during these updates.
> > 
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  include/linux/mm.h       | 37 +++++++++++++++++++++++++++++++++++++
> >  include/linux/mm_types.h |  8 +++++++-
> >  2 files changed, 44 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index c2f62bdce134..b71f2809caac 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -627,6 +627,43 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> >  	INIT_LIST_HEAD(&vma->anon_vma_chain);
> >  }
> >  
> > +/* Use when VMA is not part of the VMA tree and needs no locking */
> > +static inline void init_vm_flags(struct vm_area_struct *vma,
> > +				 unsigned long flags)
> 
> I'd suggest to make it vm_flags_init() etc.

Thinking more about it, it will be even clearer to name these vma_flags_xyz()

> Except that
> 
> Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
> 

--
Sincerely yours,
Mike.
Matthew Wilcox Jan. 26, 2023, 3:09 p.m. UTC | #5
On Thu, Jan 26, 2023 at 04:50:59PM +0200, Mike Rapoport wrote:
> On Thu, Jan 26, 2023 at 11:17:09AM +0200, Mike Rapoport wrote:
> > On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> > > +/* Use when VMA is not part of the VMA tree and needs no locking */
> > > +static inline void init_vm_flags(struct vm_area_struct *vma,
> > > +				 unsigned long flags)
> > 
> > I'd suggest to make it vm_flags_init() etc.
> 
> Thinking more about it, it will be even clearer to name these vma_flags_xyz()

Perhaps vma_VERB_flags()?

vma_init_flags()
vma_reset_flags()
vma_set_flags()
vma_clear_flags()
vma_mod_flags()
Suren Baghdasaryan Jan. 26, 2023, 4:25 p.m. UTC | #6
On Thu, Jan 26, 2023 at 7:09 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Jan 26, 2023 at 04:50:59PM +0200, Mike Rapoport wrote:
> > On Thu, Jan 26, 2023 at 11:17:09AM +0200, Mike Rapoport wrote:
> > > On Wed, Jan 25, 2023 at 12:38:46AM -0800, Suren Baghdasaryan wrote:
> > > > +/* Use when VMA is not part of the VMA tree and needs no locking */
> > > > +static inline void init_vm_flags(struct vm_area_struct *vma,
> > > > +                          unsigned long flags)
> > >
> > > I'd suggest to make it vm_flags_init() etc.
> >
> > Thinking more about it, it will be even clearer to name these vma_flags_xyz()
>
> Perhaps vma_VERB_flags()?
>
> vma_init_flags()
> vma_reset_flags()
> vma_set_flags()
> vma_clear_flags()
> vma_mod_flags()

Due to excessive email bouncing I posted the v3 of this patchset using
the original per-VMA patchset's distribution list. That might have
dropped Mike from the list. Sorry about that Mike, I'll add you to my
usual list of suspects :)
The v3 is here:
https://lore.kernel.org/all/20230125233554.153109-1-surenb@google.com/
and Andrew did suggest the same renames, so I'll be posting v4 with
those changes later today.
Thanks for the feedback!

>
diff mbox series

Patch

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c2f62bdce134..b71f2809caac 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -627,6 +627,43 @@  static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 }
 
+/* Use when VMA is not part of the VMA tree and needs no locking */
+static inline void init_vm_flags(struct vm_area_struct *vma,
+				 unsigned long flags)
+{
+	vma->vm_flags = flags;
+}
+
+/* Use when VMA is part of the VMA tree and modifications need coordination */
+static inline void reset_vm_flags(struct vm_area_struct *vma,
+				  unsigned long flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	init_vm_flags(vma, flags);
+}
+
+static inline void set_vm_flags(struct vm_area_struct *vma,
+				unsigned long flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	vma->vm_flags |= flags;
+}
+
+static inline void clear_vm_flags(struct vm_area_struct *vma,
+				  unsigned long flags)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	vma->vm_flags &= ~flags;
+}
+
+static inline void mod_vm_flags(struct vm_area_struct *vma,
+				unsigned long set, unsigned long clear)
+{
+	mmap_assert_write_locked(vma->vm_mm);
+	vma->vm_flags |= set;
+	vma->vm_flags &= ~clear;
+}
+
 static inline void vma_set_anonymous(struct vm_area_struct *vma)
 {
 	vma->vm_ops = NULL;
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2d6d790d9bed..6c7c70bf50dd 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -491,7 +491,13 @@  struct vm_area_struct {
 	 * See vmf_insert_mixed_prot() for discussion.
 	 */
 	pgprot_t vm_page_prot;
-	unsigned long vm_flags;		/* Flags, see mm.h. */
+
+	/*
+	 * Flags, see mm.h.
+	 * WARNING! Do not modify directly.
+	 * Use {init|reset|set|clear|mod}_vm_flags() functions instead.
+	 */
+	unsigned long vm_flags;
 
 	/*
 	 * For areas with an address space and backing store,