Re: [PATCH 02/13] mm: cachestat: count each page only once

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Nov 11, 2026 at 06:13:16PM +0000, Lorenzo Stoakes (ARM) wrote:
> From: Chris Mason <mason@kernel.org>
> 
> It turns out that filemap_cachestat() can unwittingly return incorrect
> page counts, if large folios are present.
> 
> This can occur due to underflow - nr_cache is nearly 2^64 on a 64-bit
> kernel if the input range starts more than 64 pages into a folio that is
> split during the call.
> 
> It can also occur due to pages after a large folio being skipped if the
> range starts in its second or later XArray slot and the call reschedules at
> that folio.
> 
> Finally it can be caused by pages being counted twice due to folios in the
> range being split or collapsed during the call.
> 
> filemap_cachestat() walks the page cache under the RCU read lock only,
> and for each entry it counts the pages that lie in the range. It does
> not track how far it has counted, so the counts are only right if the
> walk returns each index once.
> 
> But a large folio can be split during the walk, or small folios can be
> collapsed into a large one, and the next entries then cover indices that
> were already counted.
> 
> Resolve the issue by tracking the current index as one past the last index
> in the folio, and explicitly using xas_set() to set the current position in
> the XArray.
> 
> Additionally, add a check for folio_last_index reaching or exceeding
> last_index and break out of the loop if so.
> 
> Fixes: cf264e1329fb ("cachestat: implement cachestat syscall")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Chris Mason <mason@kernel.org>
> [ ljs: updated to use curr_index, fixed comments, updated commit message ]
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>