Message-ID: <CAJuCfpHR3LXfAZA_oh_2KBtFe6JQPU5T3nYMZd_ooAjRJd8xUQ@mail.gmail.com>
Date: Tue, 10 Dec 2024 09:16:23 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com, 
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Subject: Re: [PATCH v5 4/6] mm: make vma cache SLAB_TYPESAFE_BY_RCU

On Tue, Dec 10, 2024 at 8:32 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 12/10/24 17:20, Suren Baghdasaryan wrote:
> > On Tue, Dec 10, 2024 at 6:21 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> On 12/6/24 23:52, Suren Baghdasaryan wrote:
> >> > To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> >> > object reuse before RCU grace period is over will be detected inside
> >> > lock_vma_under_rcu().
> >> > lock_vma_under_rcu() enters RCU read section, finds the vma at the
> >> > given address, locks the vma and checks if it got detached or remapped
> >> > to cover a different address range. These last checks are there
> >> > to ensure that the vma was not modified after we found it but before
> >> > locking it.
> >> > vma reuse introduces several new possibilities:
> >> > 1. vma can be reused after it was found but before it is locked;
> >> > 2. vma can be reused and reinitialized (including changing its vm_mm)
> >> > while being locked in vma_start_read();
> >> > 3. vma can be reused and reinitialized after it was found but before
> >> > it is locked, then attached at a new address or to a new mm while
> >> > read-locked;
> >> > For case #1 current checks will help detecting cases when:
> >> > - vma was reused but not yet added into the tree (detached check)
> >> > - vma was reused at a different address range (address check);
> >> > We are missing the check for vm_mm to ensure the reused vma was not
> >> > attached to a different mm. This patch adds the missing check.
> >> > For case #2, we pass mm to vma_start_read() to prevent access to
> >> > unstable vma->vm_mm. This might lead to vma_start_read() returning
> >> > a false locked result but that's not critical if it's rare because
> >> > it will only lead to a retry under mmap_lock.
> >> > For case #3, we ensure the order in which vma->detached flag and
> >> > vm_start/vm_end/vm_mm are set and checked. vma gets attached after
> >> > vm_start/vm_end/vm_mm were set and lock_vma_under_rcu() should check
> >> > vma->detached before checking vm_start/vm_end/vm_mm. This is required
> >> > because attaching vma happens without vma write-lock, as opposed to
> >> > vma detaching, which requires vma write-lock. This patch adds memory
> >> > barriers inside is_vma_detached() and vma_mark_attached() needed to
> >> > order reads and writes to vma->detached vs vm_start/vm_end/vm_mm.
> >> > After these provisions, SLAB_TYPESAFE_BY_RCU is added to vm_area_cachep.
> >> > This will facilitate vm_area_struct reuse and will minimize the number
> >> > of call_rcu() calls.
> >> >
> >> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >>
> >> I'm wondering about the vma freeing path. Consider vma_complete():
> >>
> >> vma_mark_detached(vp->remove);
> >>   vma->detached = true; - plain write
> >> vm_area_free(vp->remove);
> >>   vma->vm_lock_seq = UINT_MAX; - plain write
> >>   kmem_cache_free(vm_area_cachep)
> >> ...
> >> potential reallocation
> >>
> >> against:
> >>
> >> lock_vma_under_rcu()
> >> - mas_walk finds a stale vma due to race
> >> vma_start_read()
> >>   if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence))
> >>   - can be false, the vma was not being locked on the freeing side?
> >>   down_read_trylock(&vma->vm_lock.lock) - suceeds, wasn't locked
> >>     this is acquire, but was there any release?
> >
> > Yes, there was a release. I think what you missed is that
> > vma_mark_detached() that is called from vma_complete() requires VMA to
> > be write-locked (see vma_assert_write_locked() in
> > vma_mark_detached()). The rule is that a VMA can be attached without
> > write-locking but only a write-locked VMA can be detached. So, after
>
> OK but write unlocking means the mm's seqcount is bumped and becomes
> non-equal with vma's vma->vm_lock_seq, right?
>
> Yet in the example above we happily set it to UINT_MAX and thus effectively
> false unlock it for vma_start_read()?
>
> And this is all done before the vma_complete() side would actually reach
> mmap_write_unlock(), AFAICS.

Ah, you are right. With the possibility of reuse, even a freed VMA
should be kept write-locked until it is unlocked by
mmap_write_unlock(). I think the fix for this is simply to not reset
vma->vm_lock_seq inside vm_area_free(). I'll also need to add a
comment for vm_lock_seq explaining these requirements.
Do you agree that such a change would resolve the issue?

>
> > vma_mark_detached() and before down_read_trylock(&vma->vm_lock.lock)
> > in vma_start_read() the VMA write-lock should have been released by
> > mmap_write_unlock() and therefore vma->detached=false should be
> > visible to the reader when it executed lock_vma_under_rcu().
> >
> >>   is_vma_detached() - false negative as the write above didn't propagate
> >>     here yet; a read barrier but where is the write barrier?
> >>   checks for vma->vm_mm, vm_start, vm_end - nobody reset them yet so false
> >>     positive, or they got reset on reallocation but writes didn't propagate
> >>
> >> Am I missing something that would prevent lock_vma_under_rcu() falsely
> >> succeeding here?
> >>
>

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