[Devel] [PATCH vz10 v3 1/2] mm/memcontrol: fix cache counter imbalance in mem_cgroup_replace_folio()

[Konstantin Khorenko]

On 29.07.2025 05:00, Pavel Tikhomirov wrote:
> 
> 
> On 7/29/25 00:46, Konstantin Khorenko wrote:
>> The function was checking cache flag on the new folio instead of the old one,
>> causing cache counter imbalance when replacing cache pages.
>> The new folio is freshly allocated and never has the cache flag set, so
>> we need to copy the flag from the old folio and charge the cache counter
>> accordingly.
>>
>> This fixes potential cache accounting issues during page cache replacement
>> operations like page migration between NUMA nodes.
>>
>> Another possible cause of the imbalance is the commit_charge() function
>> which unconditionally drops MEMCG_DATA_PGCACHE flag on
>> folio->memcg_data, so enhance commit_charge() to preserve the flag.
>>
>> The way of adding a new arguments was chosen in order not to miss the
>> appearance of new commit_charge() calls on rebases.
>>
>> Fixes: 6a2ca4d515c5 ("mm: Memory cgroup page cache limit")
>> https://virtuozzo.atlassian.net/browse/VSTOR-111756
>>
>> Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
>>
>> Feature: mm: Memory cgroup page cache limit
>> ---
>> v3:
>>    * enhance mem_cgroup_commit_charge() with cache parameter as well
> 
>>    * in mem_cgroup_replace_folio() put cache counter handling under
>>      !mem_cgroup_is_root() condition like other memory counters
> 
> I get why limiting memory in root memcg is kinda useless (as if allowed
> it will lead to unused memory). But limiting cache in root memcg looks
> like a completely valid operation. So I believe, we should not put cache
> accounting under !is_root.

For the record:
memcg fields:
   memory.cache.current
   memory.cache.max

are unavailable in root memcg, so let's keep all cache related counters handling under
!mem_cgroup_is_root() along with other similar counters like .memory, .memsw.

+ found one more place (uncharge_folio()) where we need to put cache accounting under the condition.