Re: [PATCH] mm/migrate: don't move memcg NR_FILE_PAGES for swapcache

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

On 11/12/26 03:48, Song Hu wrote:
>> On 11/10/26 04:09, Zi Yan wrote:
>>>
>>> It would be better to simplify the above like Joshua suggested.
>>>
>>>
>>> Like Joshua said in another email, it will be more readable if
>>> NR_SWAPCACHE updates can be moved into the then branch above.
>>
>> Can one of you write me the updated patch description? Thanks1
> 
> Sure. Here's an updated commit message incorporating Joshua's
> suggestion to describe the accounting model first and then explain why
> migration gets it wrong.
> 
> mm/migrate: don't move memcg NR_FILE_PAGES for swapcache
> 
> NR_FILE_PAGES is accounted on both the node and memcg planes for page
> cache folios.  Swapcache folios are the exception: they keep
> NR_FILE_PAGES only on the node plane, while their memcg accounting is
> tracked as NR_SWAPCACHE.
> 
> The cross-zone path in __folio_migrate_mapping() currently migrates
> NR_FILE_PAGES through mod_lruvec_state(), which updates both planes.
> This is correct for normal page cache, but wrong for swapcache folios
> because there is no memcg NR_FILE_PAGES state to move.  As a result,
> migration incorrectly increments the destination memcg counter and
> decrements the source one, leaving the per-memcg accounting incorrect
> until the memcg is destroyed.
> 
> Reproducer, on a 2-node QEMU guest with cgroup v2:
> 
>   1. create a cgroup with memory.max = 50M and swap enabled;
>   2. memset 128M of anonymous memory in it, so reclaim swaps most of
>      it out;
>   3. fork to raise the swap entry refcount, then fault the memory
>      back in, so the resident folios are swapcache ones;
>   4. migrate_pages(pid, node0 -> node1).
> 
> "file" row of memory.numa_stat (bytes) after step 4:
> 
>                     without this patch      with this patch
>                       N0          N1          N0          N1
>                      0 (*)  51691520         0           0
> 
>   (*) the source counter has gone negative; negative values read as 0.
> 
> Fix it by updating the node accounting directly for swapcache folios
> while leaving the nonexistent memcg NR_FILE_PAGES state untouched.
> Non-swapcache folios continue to use mod_lruvec_state(), preserving
> the existing behaviour.  This mirrors the existing swapcache handling
> for NR_SHMEM in the same block.  It accompanies
> commit 4eef27af2932 ("mm/migrate: move NR_FILE_THPS on cross-node
> migration") and commit c5eef2e03618 ("mm/migrate: move
> NR_KERNEL_FILE_PAGES on cross-node migration").
> 
> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Song Hu <husong@kylinos.cn>
> 
> Thanks to Joshua and Zi for the helpful reviews.

Updated, thanks!

-- 
Cheers,

David