Re: [PATCH v4] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

Yes.  The current path removes the folio from the page cache, so a later
mmap access can incur both another fault and I/O.  I should have stated
that explicitly.

For fault-around neighbors, folio_mapped() cannot distinguish the faulting
page from a speculative neighbor.  This patch takes the conservative policy
that an installed PTE represents a competing cache user and wins over the
writer's RWF_DONTCACHE hint.  It can therefore retain a speculative neighbor
which is never accessed.  Distinguishing those cases seems to require
fault-around to preserve the faulting folio while not mapping, or later
dropping, dropbehind neighbors.  I think that should be considered as a
separate follow-up.

For folio_launder(), filemap_end_dropbehind() holds the folio lock and returns
if the folio is dirty or under writeback before calling
folio_unmap_invalidate().  folio_launder() also immediately returns for a
clean folio.  I do not see how nfs_launder_folio() is reached from this
completion path unless the locked, unmapped folio can become dirty between
those checks.  Is there a path I am missing?