Re: [PATCH v5 4/6] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Suren Baghdasaryan]

On Tue, Dec 10, 2024 at 6:21 AM Vlastimil Babka <vbabka@xxxxxxx> 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@xxxxxxxxxx>
>
> 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
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?
>