[PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Suren Baghdasaryan]

oom-reaper and process_mrelease system call should protect against
races with exit_mmap which can destroy page tables while they
walk the VMA tree. oom-reaper protects from that race by setting
MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
before taking and releasing mmap_write_lock. process_mrelease has
to elevate mm->mm_users to prevent such race. Both oom-reaper and
process_mrelease hold mmap_read_lock when walking the VMA tree.
The locking rules and mechanisms could be simpler if exit_mmap takes
mmap_write_lock while executing destructive operations such as
free_pgtables.
Change exit_mmap to hold the mmap_write_lock when calling
free_pgtables. Operations like unmap_vmas() and unlock_range() are not
destructive and could run under mmap_read_lock but for simplicity we
take one mmap_write_lock during almost the entire operation. Note
also that because oom-reaper checks VM_LOCKED flag, unlock_range()
should not be allowed to race with it.
In most cases this lock should be uncontended. Previously, Kirill
reported ~4% regression caused by a similar change [1]. We reran the
same test and although the individual results are quite noisy, the
percentiles show lower regression with 1.6% being the worst case [2].
The change allows oom-reaper and process_mrelease to execute safely
under mmap_read_lock without worries that exit_mmap might destroy page
tables from under them.

[1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
[2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
changes in v2
- Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox

 mm/mmap.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index bfb0ea164a90..f4e09d390a07 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
 		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
 		 * __oom_reap_task_mm() will not block.
 		 *
-		 * This needs to be done before calling munlock_vma_pages_all(),
+		 * This needs to be done before calling unlock_range(),
 		 * which clears VM_LOCKED, otherwise the oom reaper cannot
 		 * reliably test it.
 		 */
 		(void)__oom_reap_task_mm(mm);
 
 		set_bit(MMF_OOM_SKIP, &mm->flags);
-		mmap_write_lock(mm);
-		mmap_write_unlock(mm);
 	}
 
+	mmap_write_lock(mm);
 	if (mm->locked_vm)
 		unlock_range(mm->mmap, ULONG_MAX);
 
 	arch_exit_mmap(mm);
 
 	vma = mm->mmap;
-	if (!vma)	/* Can happen if dup_mmap() received an OOM */
+	if (!vma) {
+		/* Can happen if dup_mmap() received an OOM */
+		mmap_write_unlock(mm);
 		return;
+	}
 
 	lru_add_drain();
 	flush_cache_mm(mm);
@@ -3171,16 +3173,14 @@ void exit_mmap(struct mm_struct *mm)
 	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb);
 
-	/*
-	 * Walk the list again, actually closing and freeing it,
-	 * with preemption enabled, without holding any MM locks.
-	 */
+	/* Walk the list again, actually closing and freeing it. */
 	while (vma) {
 		if (vma->vm_flags & VM_ACCOUNT)
 			nr_accounted += vma_pages(vma);
 		vma = remove_vma(vma);
 		cond_resched();
 	}
+	mmap_write_unlock(mm);
 	vm_unacct_memory(nr_accounted);
 }
 
-- 
2.34.0.rc2.393.gf8c9666880-goog

[PATCH v2 2/2] mm/oom_kill: allow process_mrelease to run under mmap_lock protection

[Suren Baghdasaryan]

With exit_mmap holding mmap_write_lock during free_pgtables call,
process_mrelease does not need to elevate mm->mm_users in order to
prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
is walking the VMA tree. The change prevents process_mrelease from
calling the last mmput, which can lead to waiting for IO completion
in exit_aio.

Fixes: 337546e83fc7 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/oom_kill.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1ddabefcfb5a..67780386f478 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -1169,15 +1169,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 		goto put_task;
 	}
 
-	if (mmget_not_zero(p->mm)) {
-		mm = p->mm;
-		if (task_will_free_mem(p))
-			reap = true;
-		else {
-			/* Error only if the work has not been done already */
-			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
-				ret = -EINVAL;
-		}
+	mm = p->mm;
+	mmgrab(mm);
+
+	if (task_will_free_mem(p))
+		reap = true;
+	else {
+		/* Error only if the work has not been done already */
+		if (!test_bit(MMF_OOM_SKIP, &mm->flags))
+			ret = -EINVAL;
 	}
 	task_unlock(p);
 
@@ -1188,13 +1188,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
 		ret = -EINTR;
 		goto drop_mm;
 	}
-	if (!__oom_reap_task_mm(mm))
+	/*
+	 * Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
+	 * possible change in exit_mmap is seen
+	 */
+	if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
 		ret = -EAGAIN;
 	mmap_read_unlock(mm);
 
 drop_mm:
-	if (mm)
-		mmput(mm);
+	mmdrop(mm);
 put_task:
 	put_task_struct(task);
 	return ret;
-- 
2.34.0.rc2.393.gf8c9666880-goog

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Michal Hocko]

On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> oom-reaper and process_mrelease system call should protect against
> races with exit_mmap which can destroy page tables while they
> walk the VMA tree. oom-reaper protects from that race by setting
> MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> before taking and releasing mmap_write_lock. process_mrelease has
> to elevate mm->mm_users to prevent such race. Both oom-reaper and
> process_mrelease hold mmap_read_lock when walking the VMA tree.
> The locking rules and mechanisms could be simpler if exit_mmap takes
> mmap_write_lock while executing destructive operations such as
> free_pgtables.
> Change exit_mmap to hold the mmap_write_lock when calling
> free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> destructive and could run under mmap_read_lock but for simplicity we
> take one mmap_write_lock during almost the entire operation. Note
> also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> should not be allowed to race with it.
> In most cases this lock should be uncontended. Previously, Kirill
> reported ~4% regression caused by a similar change [1]. We reran the
> same test and although the individual results are quite noisy, the
> percentiles show lower regression with 1.6% being the worst case [2].
> The change allows oom-reaper and process_mrelease to execute safely
> under mmap_read_lock without worries that exit_mmap might destroy page
> tables from under them.
> 
> [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> changes in v2
> - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> 
>  mm/mmap.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bfb0ea164a90..f4e09d390a07 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
>  		 * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
>  		 * __oom_reap_task_mm() will not block.
>  		 *
> -		 * This needs to be done before calling munlock_vma_pages_all(),
> +		 * This needs to be done before calling unlock_range(),
>  		 * which clears VM_LOCKED, otherwise the oom reaper cannot
>  		 * reliably test it.
>  		 */
>  		(void)__oom_reap_task_mm(mm);
>  
>  		set_bit(MMF_OOM_SKIP, &mm->flags);

Why do you keep this in place?

Other than that looks OK to me. Maybe we want to add an explicit note
that vm_ops::close cannot take mmap_sem in any form. The changelog
should also mention that you have considered remove_vma and its previous
no MM locking assumption. You can argue that fput is async and close
callback shouldn't really need mmap_sem.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Suren Baghdasaryan]

On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> > oom-reaper and process_mrelease system call should protect against
> > races with exit_mmap which can destroy page tables while they
> > walk the VMA tree. oom-reaper protects from that race by setting
> > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> > before taking and releasing mmap_write_lock. process_mrelease has
> > to elevate mm->mm_users to prevent such race. Both oom-reaper and
> > process_mrelease hold mmap_read_lock when walking the VMA tree.
> > The locking rules and mechanisms could be simpler if exit_mmap takes
> > mmap_write_lock while executing destructive operations such as
> > free_pgtables.
> > Change exit_mmap to hold the mmap_write_lock when calling
> > free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> > destructive and could run under mmap_read_lock but for simplicity we
> > take one mmap_write_lock during almost the entire operation. Note
> > also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> > should not be allowed to race with it.
> > In most cases this lock should be uncontended. Previously, Kirill
> > reported ~4% regression caused by a similar change [1]. We reran the
> > same test and although the individual results are quite noisy, the
> > percentiles show lower regression with 1.6% being the worst case [2].
> > The change allows oom-reaper and process_mrelease to execute safely
> > under mmap_read_lock without worries that exit_mmap might destroy page
> > tables from under them.
> >
> > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > changes in v2
> > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> >
> >  mm/mmap.c | 16 ++++++++--------
> >  1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index bfb0ea164a90..f4e09d390a07 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
> >                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> >                * __oom_reap_task_mm() will not block.
> >                *
> > -              * This needs to be done before calling munlock_vma_pages_all(),
> > +              * This needs to be done before calling unlock_range(),
> >                * which clears VM_LOCKED, otherwise the oom reaper cannot
> >                * reliably test it.
> >                */
> >               (void)__oom_reap_task_mm(mm);
> >
> >               set_bit(MMF_OOM_SKIP, &mm->flags);
>
> Why do you keep this in place?

Sorry for the delay, I was out last week.
I missed your comment about removing MMF_OOM_SKIP at
https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz
I'll look into removing it in a separate patch, which I think would be cleaner.

>
> Other than that looks OK to me. Maybe we want to add an explicit note
> that vm_ops::close cannot take mmap_sem in any form. The changelog
> should also mention that you have considered remove_vma and its previous
> no MM locking assumption. You can argue that fput is async and close
> callback shouldn't really need mmap_sem.

Should I post another version of this patch with the patch description
clarifying these points and additional comments as you suggested?

> --
> Michal Hocko
> SUSE Labs

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Matthew Wilcox]

On Mon, Dec 06, 2021 at 10:35:03AM -0800, Suren Baghdasaryan wrote:
> > Other than that looks OK to me. Maybe we want to add an explicit note
> > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > should also mention that you have considered remove_vma and its previous
> > no MM locking assumption. You can argue that fput is async and close
> > callback shouldn't really need mmap_sem.
> 
> Should I post another version of this patch with the patch description
> clarifying these points and additional comments as you suggested?

fyi, vm_ops->close() is already called with the mmap_sem held for write
in __split_vma().  If that needs to be documented, it's a separate patch
because it's absolutely not a consequence of this patch.

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Michal Hocko]

On Mon 06-12-21 18:52:28, Matthew Wilcox wrote:
> On Mon, Dec 06, 2021 at 10:35:03AM -0800, Suren Baghdasaryan wrote:
> > > Other than that looks OK to me. Maybe we want to add an explicit note
> > > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > > should also mention that you have considered remove_vma and its previous
> > > no MM locking assumption. You can argue that fput is async and close
> > > callback shouldn't really need mmap_sem.
> > 
> > Should I post another version of this patch with the patch description
> > clarifying these points and additional comments as you suggested?
> 
> fyi, vm_ops->close() is already called with the mmap_sem held for write
> in __split_vma().  If that needs to be documented, it's a separate patch
> because it's absolutely not a consequence of this patch.

Agreed! We definitely want to document that.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Michal Hocko]

On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote:
> On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> > > oom-reaper and process_mrelease system call should protect against
> > > races with exit_mmap which can destroy page tables while they
> > > walk the VMA tree. oom-reaper protects from that race by setting
> > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> > > before taking and releasing mmap_write_lock. process_mrelease has
> > > to elevate mm->mm_users to prevent such race. Both oom-reaper and
> > > process_mrelease hold mmap_read_lock when walking the VMA tree.
> > > The locking rules and mechanisms could be simpler if exit_mmap takes
> > > mmap_write_lock while executing destructive operations such as
> > > free_pgtables.
> > > Change exit_mmap to hold the mmap_write_lock when calling
> > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> > > destructive and could run under mmap_read_lock but for simplicity we
> > > take one mmap_write_lock during almost the entire operation. Note
> > > also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> > > should not be allowed to race with it.
> > > In most cases this lock should be uncontended. Previously, Kirill
> > > reported ~4% regression caused by a similar change [1]. We reran the
> > > same test and although the individual results are quite noisy, the
> > > percentiles show lower regression with 1.6% being the worst case [2].
> > > The change allows oom-reaper and process_mrelease to execute safely
> > > under mmap_read_lock without worries that exit_mmap might destroy page
> > > tables from under them.
> > >
> > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > > changes in v2
> > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> > >
> > >  mm/mmap.c | 16 ++++++++--------
> > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index bfb0ea164a90..f4e09d390a07 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
> > >                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> > >                * __oom_reap_task_mm() will not block.
> > >                *
> > > -              * This needs to be done before calling munlock_vma_pages_all(),
> > > +              * This needs to be done before calling unlock_range(),
> > >                * which clears VM_LOCKED, otherwise the oom reaper cannot
> > >                * reliably test it.
> > >                */
> > >               (void)__oom_reap_task_mm(mm);
> > >
> > >               set_bit(MMF_OOM_SKIP, &mm->flags);
> >
> > Why do you keep this in place?
> 
> Sorry for the delay, I was out last week.
> I missed your comment about removing MMF_OOM_SKIP at
> https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz
> I'll look into removing it in a separate patch, which I think would be cleaner.

The point of this code was to sync up the oom_repaer and exit_mmap. Now
that your patch uses proper locking for that to happen then MMF_OOM_SKIP
is not really necessary. IIRC all you need to guarantee is that the vma
tree is empty when exit_mmap does all its work - i.e set mm->mmap to
NULL. You can do that after remove_vma loop but it would be equally safe
at any time after vma = mm->mmap as the loop relies on the vma chain.
Doing that after would be slightly nicer if you ask me.
 
> > Other than that looks OK to me. Maybe we want to add an explicit note
> > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > should also mention that you have considered remove_vma and its previous
> > no MM locking assumption. You can argue that fput is async and close
> > callback shouldn't really need mmap_sem.
> 
> Should I post another version of this patch with the patch description
> clarifying these points and additional comments as you suggested?

Yes please.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Suren Baghdasaryan]

On Tue, Dec 7, 2021 at 2:10 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote:
> > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> > > > oom-reaper and process_mrelease system call should protect against
> > > > races with exit_mmap which can destroy page tables while they
> > > > walk the VMA tree. oom-reaper protects from that race by setting
> > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> > > > before taking and releasing mmap_write_lock. process_mrelease has
> > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and
> > > > process_mrelease hold mmap_read_lock when walking the VMA tree.
> > > > The locking rules and mechanisms could be simpler if exit_mmap takes
> > > > mmap_write_lock while executing destructive operations such as
> > > > free_pgtables.
> > > > Change exit_mmap to hold the mmap_write_lock when calling
> > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> > > > destructive and could run under mmap_read_lock but for simplicity we
> > > > take one mmap_write_lock during almost the entire operation. Note
> > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> > > > should not be allowed to race with it.
> > > > In most cases this lock should be uncontended. Previously, Kirill
> > > > reported ~4% regression caused by a similar change [1]. We reran the
> > > > same test and although the individual results are quite noisy, the
> > > > percentiles show lower regression with 1.6% being the worst case [2].
> > > > The change allows oom-reaper and process_mrelease to execute safely
> > > > under mmap_read_lock without worries that exit_mmap might destroy page
> > > > tables from under them.
> > > >
> > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > ---
> > > > changes in v2
> > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> > > >
> > > >  mm/mmap.c | 16 ++++++++--------
> > > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > index bfb0ea164a90..f4e09d390a07 100644
> > > > --- a/mm/mmap.c
> > > > +++ b/mm/mmap.c
> > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
> > > >                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> > > >                * __oom_reap_task_mm() will not block.
> > > >                *
> > > > -              * This needs to be done before calling munlock_vma_pages_all(),
> > > > +              * This needs to be done before calling unlock_range(),
> > > >                * which clears VM_LOCKED, otherwise the oom reaper cannot
> > > >                * reliably test it.
> > > >                */
> > > >               (void)__oom_reap_task_mm(mm);
> > > >
> > > >               set_bit(MMF_OOM_SKIP, &mm->flags);
> > >
> > > Why do you keep this in place?
> >
> > Sorry for the delay, I was out last week.
> > I missed your comment about removing MMF_OOM_SKIP at
> > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz
> > I'll look into removing it in a separate patch, which I think would be cleaner.
>
> The point of this code was to sync up the oom_repaer and exit_mmap. Now
> that your patch uses proper locking for that to happen then MMF_OOM_SKIP
> is not really necessary. IIRC all you need to guarantee is that the vma
> tree is empty when exit_mmap does all its work - i.e set mm->mmap to
> NULL. You can do that after remove_vma loop but it would be equally safe
> at any time after vma = mm->mmap as the loop relies on the vma chain.
> Doing that after would be slightly nicer if you ask me.

Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP
as a separate patch. This patchset has already been extensively tested
and it will be easier for me to test MMF_OOM_SKIP removal separately.

>
> > > Other than that looks OK to me. Maybe we want to add an explicit note
> > > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > > should also mention that you have considered remove_vma and its previous
> > > no MM locking assumption. You can argue that fput is async and close
> > > callback shouldn't really need mmap_sem.
> >
> > Should I post another version of this patch with the patch description
> > clarifying these points and additional comments as you suggested?
>
> Yes please.

Will re-post. So, to clarify, we want:
- Patch description to include explanation that remove_vma is now
being called under MM lock but this should not be a problem because
fput and vm_ops->close do not and should not take mmap_sem.
- Add a comment for vm_ops->close that the callback should not take
mmap_sem, with explanation that __split_vma and exit_mmap use this
callback with the mmap_sem write lock taken.
Is that correct?

>
> --
> Michal Hocko
> SUSE Labs

Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Suren Baghdasaryan]

On Tue, Dec 7, 2021 at 8:50 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Tue, Dec 7, 2021 at 2:10 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote:
> > > On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> > > > > oom-reaper and process_mrelease system call should protect against
> > > > > races with exit_mmap which can destroy page tables while they
> > > > > walk the VMA tree. oom-reaper protects from that race by setting
> > > > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> > > > > before taking and releasing mmap_write_lock. process_mrelease has
> > > > > to elevate mm->mm_users to prevent such race. Both oom-reaper and
> > > > > process_mrelease hold mmap_read_lock when walking the VMA tree.
> > > > > The locking rules and mechanisms could be simpler if exit_mmap takes
> > > > > mmap_write_lock while executing destructive operations such as
> > > > > free_pgtables.
> > > > > Change exit_mmap to hold the mmap_write_lock when calling
> > > > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> > > > > destructive and could run under mmap_read_lock but for simplicity we
> > > > > take one mmap_write_lock during almost the entire operation. Note
> > > > > also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> > > > > should not be allowed to race with it.
> > > > > In most cases this lock should be uncontended. Previously, Kirill
> > > > > reported ~4% regression caused by a similar change [1]. We reran the
> > > > > same test and although the individual results are quite noisy, the
> > > > > percentiles show lower regression with 1.6% being the worst case [2].
> > > > > The change allows oom-reaper and process_mrelease to execute safely
> > > > > under mmap_read_lock without worries that exit_mmap might destroy page
> > > > > tables from under them.
> > > > >
> > > > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> > > > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> > > > >
> > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > > ---
> > > > > changes in v2
> > > > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> > > > >
> > > > >  mm/mmap.c | 16 ++++++++--------
> > > > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > > > >
> > > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > > index bfb0ea164a90..f4e09d390a07 100644
> > > > > --- a/mm/mmap.c
> > > > > +++ b/mm/mmap.c
> > > > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
> > > > >                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> > > > >                * __oom_reap_task_mm() will not block.
> > > > >                *
> > > > > -              * This needs to be done before calling munlock_vma_pages_all(),
> > > > > +              * This needs to be done before calling unlock_range(),
> > > > >                * which clears VM_LOCKED, otherwise the oom reaper cannot
> > > > >                * reliably test it.
> > > > >                */
> > > > >               (void)__oom_reap_task_mm(mm);
> > > > >
> > > > >               set_bit(MMF_OOM_SKIP, &mm->flags);
> > > >
> > > > Why do you keep this in place?
> > >
> > > Sorry for the delay, I was out last week.
> > > I missed your comment about removing MMF_OOM_SKIP at
> > > https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz
> > > I'll look into removing it in a separate patch, which I think would be cleaner.
> >
> > The point of this code was to sync up the oom_repaer and exit_mmap. Now
> > that your patch uses proper locking for that to happen then MMF_OOM_SKIP
> > is not really necessary. IIRC all you need to guarantee is that the vma
> > tree is empty when exit_mmap does all its work - i.e set mm->mmap to
> > NULL. You can do that after remove_vma loop but it would be equally safe
> > at any time after vma = mm->mmap as the loop relies on the vma chain.
> > Doing that after would be slightly nicer if you ask me.
>
> Will do. But if you don't mind I'll post the removal of MMF_OOM_SKIP
> as a separate patch. This patchset has already been extensively tested
> and it will be easier for me to test MMF_OOM_SKIP removal separately.
>
> >
> > > > Other than that looks OK to me. Maybe we want to add an explicit note
> > > > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > > > should also mention that you have considered remove_vma and its previous
> > > > no MM locking assumption. You can argue that fput is async and close
> > > > callback shouldn't really need mmap_sem.
> > >
> > > Should I post another version of this patch with the patch description
> > > clarifying these points and additional comments as you suggested?
> >
> > Yes please.
>
> Will re-post. So, to clarify, we want:
> - Patch description to include explanation that remove_vma is now
> being called under MM lock but this should not be a problem because
> fput and vm_ops->close do not and should not take mmap_sem.
> - Add a comment for vm_ops->close that the callback should not take
> mmap_sem, with explanation that __split_vma and exit_mmap use this
> callback with the mmap_sem write lock taken.
> Is that correct?

Assuming my understanding was correct, posted v3 at
https://lore.kernel.org/all/20211207215031.2251719-1-surenb@google.com/

>
> >
> > --
> > Michal Hocko
> > SUSE Labs