Re: [PATCH 05/46] mm/truncate: look again if a folio left the mapping

[Kiryl Shutsemau]

On Wed, Oct 28, 2026 at 10:45:34PM +0100, David Hildenbrand (Arm) wrote:
> My brain hurts, does the above make sense?

Yes, I missed that the loop only goes back to start when a lookup
comes back empty below end. When the last batch reaches end, it exits
without another look. It doesn't need the split either: any punch hole
whose last page is in the page cache gets there.

That exit looks unintended to me. b9a8a4195c7d ("truncate,shmem: Handle
truncates that split large folios") turned the "if (start >= end) goto
out" early exit into the loop condition. Before it, the loop went back
to start until a lookup from start found nothing; Hugh kept that on
purpose for hole punch in 792ceaefe621 ("mm/fs: fix pessimization in
hole-punching pagecache").

Restoring it is enough, and makes the "restart to make sure all gone"
comment true again:

+       if (start >= end)
+               return;
+
        index = start;
-       while (index < end) {
+       for (;;) {

Then a replaced folio is found on the next round, and truncate needs no
special case for it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov