Message-ID: <CAJuCfpHcweCby61FpVr8cAhcr1JkXQyZ_LbKL3HrM8fsFTGiog@mail.gmail.com>
Date: Wed, 11 Dec 2024 07:30:05 -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 3:01 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Tue, Dec 10, 2024 at 9:25 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >
> > On 12/10/24 18:16, Suren Baghdasaryan wrote:
> > > 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
> >
> > But even if we don't reset vm_lock_seq to UINT_MAX, then whover reallocated
> > it can proceed and end up doing a vma_start_write() and rewrite it there
> > anyway, no?
>
> Actually, I think with a small change we can simplify these locking rules:
>
> static inline void vma_start_write(struct vm_area_struct *vma)
> {
>         int mm_lock_seq;
>
> -        if (__is_vma_write_locked(vma, &mm_lock_seq))
> -                return;
> +        mmap_assert_write_locked(vma->vm_mm);
> +        mm_lock_seq = vma->vm_mm->mm_lock_seq;
>
>         down_write(&vma->vm_lock->lock);
>         /*
>         * We should use WRITE_ONCE() here because we can have concurrent reads
>         * from the early lockless pessimistic check in vma_start_read().
>         * We don't really care about the correctness of that early check, but
>         * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy.
>         */
>         WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
>         up_write(&vma->vm_lock->lock);
> }
>
> This will force vma_start_write() to always write-lock vma->vm_lock
> before changing vma->vm_lock_seq. Since vma->vm_lock survives reuse,
> the other readers/writers will synchronize on it even if vma got
> reused.

After thinking of all the alternatives, I think the cleanest way to
handle vma detaching would be to follow the same pattern as for vma
attaching. To attach a vma we do:

vma->vm_mm = xxx;
...
vma_mark_attached()
    smp_wmb();
    WRITE_ONCE(vma->detached, false);


lock_vma_under_rcu() ensures that a vma is attached and still
unchanged like this:

lock_vma_under_rcu()
    vma_start_read();
    is_vma_detached()
        detached = READ_ONCE(vma->detached);
        smp_rmb();
    if (vma->vm_mm != mm)

So, vm_area_free() can follow the same pattern to ensure vma reuse
gets detected even if lock_vma_under_rcu() succeeds in locking the
vma:

vm_area_free()
    vma->vm_mm = NULL;
    smp_wmb();
    WRITE_ONCE(vma->detached, true);

Vlastimil, I think that should address the race you described. WDYT?

>
> >
> > > 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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