Re: [PATCH v2] mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in collapse_file()

[Johannes Weiner]

On Thu, Jan 29, 2026 at 08:29:25PM -0800, Shakeel Butt wrote:
> In META's fleet, we observed high-level cgroups showing zero file memcg
> stats while their descendants had non-zero values. Investigation using
> drgn revealed that these parent cgroups actually had negative file stats,
> aggregated from their children.
> 
> This issue became more frequent after deploying thp-always more widely,
> pointing to a correlation with THP file collapsing. The root cause is
> that collapse_file() assumes old folios and the new THP belong to the
> same node and memcg. When this assumption breaks, stats become skewed.
> The bug affects not just memcg stats but also per-numa stats, and not
> just NR_FILE_PAGES but also NR_SHMEM.
> 
> The assumption breaks in scenarios such as:
> 
> 1. Small folios allocated on one node while the THP gets allocated on a
>    different node.
> 
> 2. A package downloader running in one cgroup populates the page cache,
>    while a job in a different cgroup executes the downloaded binary.
> 
> 3. A file shared between processes in different cgroups, where one
>    process faults in the pages and khugepaged (or madvise(COLLAPSE))
>    collapses them on behalf of the other.
> 
> Fix the accounting by explicitly incrementing stats for the new THP and
> decrementing stats for the old folios being replaced.
> 
> Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>