Re: [PATCH] mm/migrate: move NR_FILE_THPS on cross-node migration

[Garg, Shivank]

On Mon, 2026-10-26 at 11:59 +0100, David Hildenbrand (Arm) wrote:
> On 10/25/26 12:02, Ridong Chen wrote:
> > From: Ridong Chen <chenridong@xxxxxxxxxx>
> > 
> > When a page cache folio is migrated to another node,
> > __folio_migrate_mapping() moves the per-node and per-memcg counters
> > that are keyed off the folio's node: NR_FILE_PAGES, NR_SHMEM,
> > NR_SHMEM_THPS, NR_SWAPCACHE and NR_FILE_DIRTY.  NR_FILE_THPS is not
> > moved, although __filemap_add_folio() charges it to the source node and
> > filemap_unaccount_folio() takes it off whatever node the folio lives on
> > when it is removed.
> > 
> > Commit 0b52c420350e ("mm: fix shmem THP counters on migration") added
> > the NR_SHMEM_THPS case but not its file-backed counterpart.  Now that
> > ext4, xfs, btrfs and erofs can all have PMD-sized page cache folios,
> > migrating them via NUMA balancing, mbind(), migratepages, demotion or
> > memory offlining leaks FileHugePages on the source node and underflows
> > it on the destination, in both /sys/devices/system/node/nodeN/meminfo
> > and memory.numa_stat.  The global value is unaffected, which is why this
> > went unnoticed.
> > 
> > Reproducer, on a 2-node QEMU guest with THP set to "always":
> > 
> >   1. read a 256M ext4 file through a MADV_HUGEPAGE mapping while bound
> >      to node 0, so its page cache is 128 PMD-sized folios on node 0;
> >   2. move the mapping to node 1 with mbind(MPOL_MF_MOVE);
> >   3. unmap the file and drop its page cache.
> > 
> > nr_file_hugepages in /sys/devices/system/node/nodeN/vmstat after each
> > step (in units of PMD-sized folios):
> > 
> >                     without this patch     with this patch
> >                      node0     node1       node0     node1
> >   1. cached           128         0         128         0
> >   2. migrated         128         0           0       128
> >   3. dropped          128         0 (*)       0         0
> > 
> >   (*) the counter has underflowed to -128; negative values read as 0.
> > 
> > Without the patch the 128 folios stay accounted to node 0 forever, and
> > every further round adds another 128.  The same happens to the per-memcg
> > per-node counter: after step 3, memory.stat of the cgroup shows
> > "file_thp 0", while memory.numa_stat still shows
> > "file_thp N0=268435456 N1=0".  With the patch both are 0.
> > 
> > Move NR_FILE_THPS together with NR_FILE_PAGES, using the same condition
> > filemap_unaccount_folio() uses to take it off, so that THPs in the swap
> > cache are left alone.
> > 
> > Fixes: d68eccad3706 ("mm/filemap: Allow large folios to be added to the page cache")
> > Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> > ---
> > Note: prepared with help from an LLM-based assistant (Kiro, claude-opus-5.5).
> > 
> >  mm/migrate.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 7bdcdb57652f..d55f576a2579 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -700,6 +700,10 @@ static int __folio_migrate_mapping(struct address_space *mapping,
> >  				mod_lruvec_state(old_lruvec, NR_SHMEM_THPS, -nr);
> >  				mod_lruvec_state(new_lruvec, NR_SHMEM_THPS, nr);
> >  			}
> > +		} else if (!folio_test_swapbacked(folio) &&
> > +			   folio_test_pmd_mappable(folio)) {
> > +			mod_lruvec_state(old_lruvec, NR_FILE_THPS, -nr);
> > +			mod_lruvec_state(new_lruvec, NR_FILE_THPS, nr);
> >  		}
> 
> Looking at filemap_unaccount_folio(), I was (once again) surprise about the
> different handling.
> 
> Key is that we might be migration aonymous folios in the swapcache.
> 
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> 

filemap_unaccount_folio() also checks for AS_KERNEL_FILE, but
__folio_migrate_mapping() does not.

Should add this as well?

Thanks,
Shivank