Re: [PATCH v7 16/17] mm: make vma cache SLAB_TYPESAFE_BY_RCU

Vlastimil Babka
2025-01-08T19:01:03Z

On 1/8/25 19:44, Suren Baghdasaryan wrote:
> On Wed, Jan 8, 2025 at 10:21 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 12/26/24 18:07, 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 by
>> > lock_vma_under_rcu(). Current checks are sufficient as long as vma
>> > is detached before it is freed. Implement this guarantee by calling
>> > vma_ensure_detached() before vma is freed and make vm_area_cachep
>> > SLAB_TYPESAFE_BY_RCU. This will facilitate vm_area_struct reuse and
>> > will minimize the number of call_rcu() calls.
>> >
>> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>
>> I've noticed vm_area_dup() went back to the approach of "we memcpy
>> everything including vma_lock and detached (now the vm_refcnt) followed by a
>> vma_init_lock(..., true) that does refcount_set(&vma->vm_refcnt, 0);
>> Is that now safe against a racing lock_vma_under_rcu()? I think it's not?
>
> I think it's safe because vma created by vm_area_dup() is not in the
> vma tree yet, so lock_vma_under_rcu() does not see it until it's added
> into the tree. Note also that at the time when the new vma gets added
> into the tree, the vma has to be write-locked
> (vma_iter_store()->vma_mark_attached()->vma_assert_write_locked()).
> So, lock_vma_under_rcu() won't use the new vma even after it's added
> into the tree until we unlock the vma.

What about something like this, where vma starts out as attached as thus
reachable:

A: B: C:
lock_vma_under_rcu()
vma = mas_walk()
vma_start_read()
vm_lock_seq == mm->mm_lock_seq.sequence

vma detached and freed

vm_area_dup()
- vma reallocated
- memcpy() copies non-zero refcnt from orig

__refcount_inc_not_zero_limited() succeeds

vma_init_lock();
refcount_set(&vma->vm_refcnt, 0);

- vm_lock_seq validation fails (could it even succeed?)
vma_refcount_put(vma);
__refcount_dec_and_test makes refcount -1