Re: [PATCH v2 0/3] use vma locks for proc/pid/{smaps|numa_maps} reads

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/13/26 05:58, Suren Baghdasaryan wrote:
> On Tue, May 12, 2026 at 2:28 AM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
>>
>> On 4/26/26 08:27, Suren Baghdasaryan wrote:
>>> Use per-vma locks when reading /proc/pid/smaps and /proc/pid/numa_maps
>>> similar to /proc/pid/maps to reduce contention on central mmap_lock. One
>>> major difference between maps and smaps/numa_maps reading is that the
>>> latter executes page table walk which can't be done under RCU due to a
>>> possibility of sleeping. Therefore we drop RCU read lock before this walk
>>> while keeping the VMA locked. After the walk we retake RCU read lock,
>>> reset VMA iterator and proceed with the next VMA.
>>
>> With many small VMAs, is that overhead noticable?
> 
> It might be but the point of this patchset (and the previous one that
> made a similar change for /proc/pid/maps) is to reduce mmap_lock
> contention, not to speed up the read operation, which is not a
> performance critical part.

Well, this interface has been around .. forever, so if there is a noticeable
change in performance it should be called out.

> The original problem that Paul McKenney
> described and which kicked these series of changes is that a
> low-priority monitoring process reading /proc/pid/{maps|smaps|...} can
> block a high-priority updates by holding the mmap_lock. You can see
> details about this problem and the numbers Paul obtained with the
> previous change in here:
> https://lore.kernel.org/all/20250719182854.3166724-1-surenb@google.com/

Yes, I know. A problem that has been around ... forever as well :)

-- 
Cheers,

David