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

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

On 11/10/26 04:09, Zi Yan wrote:
> On Tue Nov 3, 2026 at 11:43 PM EST, Song Hu wrote:
>> NR_FILE_PAGES lives on both the node and the memcg plane for page
>> cache folios: filemap_add_folio() and filemap_unaccount_folio() charge
>> it to the folio's node and memcg, while __swap_cache_add_folio() and
>> __swap_cache_del_folio() maintain the node plane of NR_FILE_PAGES and
>> the memcg plane of NR_SWAPCACHE.  A folio in the swap cache therefore
>> carries no per-memcg NR_FILE_PAGES count: shmem folios drop their
>> memcg file accounting when swapped out, and anon folios never had any.
>>
>> The cross-zone arm of __folio_migrate_mapping() moves NR_FILE_PAGES on
>> both planes unconditionally.  For swapcache folios the memcg plane is
>> empty, so migrating one misaccounts it: the destination accrues
>> nr_pages that were never accounted there while the source counter
>> underflows.  The wrong counts persist until the memcg is destroyed,
>> but the aggregate in memory.stat stays correct, so the drift is easy
>> to miss.  NUMA balancing, migratepages, compaction and memory hotplug
>> can all migrate such folios.
> 
> It would be better to simplify the above like Joshua suggested.
> 
>>
>> 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.
>>
>> Keep moving the node plane via mod_node_page_state() and skip only
>> the empty memcg plane, mirroring the swapcache exclusion the NR_SHMEM
>> handling in the same block already applies.  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").
>>
>> Fixes: 0d1c20722ab3 ("mm: memcontrol: switch to native NR_FILE_PAGES and NR_SHMEM counters")
>> Signed-off-by: Song Hu <husong@kylinos.cn>
>> ---
>>  mm/migrate.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 28a8d7734735..bc60f6587d1b 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -690,8 +690,20 @@ static int __folio_migrate_mapping(struct address_space *mapping,
>>  		old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
>>  		new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);
>>  
>> -		mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr);
>> -		mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr);
>> +		/*
>> +		 * Swapcache folios carry NR_FILE_PAGES on the node plane
>> +		 * only (their memcg footprint is NR_SWAPCACHE, see
>> +		 * swap_state.c), so move only the node plane for them.
>> +		 */
>> +		if (folio_test_swapcache(folio)) {
>> +			mod_node_page_state(oldzone->zone_pgdat, NR_FILE_PAGES,
>> +					    -nr);
>> +			mod_node_page_state(newzone->zone_pgdat, NR_FILE_PAGES,
>> +					    nr);
>> +		} else {
>> +			mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr);
>> +			mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr);
>> +		}
> 
> 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

-- 
Cheers,

David