Re: [PATCH 6.17.y] mm/truncate: unmap large folio on split failure

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Nov 21, 2025 at 02:17:05PM +0000, Kiryl Shutsemau wrote:
> On Fri, Nov 21, 2025 at 01:20:08PM +0000, Kiryl Shutsemau wrote:
> > On Fri, Nov 21, 2025 at 10:46:11AM +0100, Greg KH wrote:
> > > On Thu, Nov 20, 2025 at 04:52:21PM +0000, Kiryl Shutsemau wrote:
> > > > Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
> > > > supposed to generate SIGBUS.
> > > > 
> > > > This behavior might not be respected on truncation.
> > > > 
> > > > During truncation, the kernel splits a large folio in order to reclaim
> > > > memory.  As a side effect, it unmaps the folio and destroys PMD mappings
> > > > of the folio.  The folio will be refaulted as PTEs and SIGBUS semantics
> > > > are preserved.
> > > > 
> > > > However, if the split fails, PMD mappings are preserved and the user will
> > > > not receive SIGBUS on any accesses within the PMD.
> > > > 
> > > > Unmap the folio on split failure.  It will lead to refault as PTEs and
> > > > preserve SIGBUS semantics.
> > > > 
> > > > Make an exception for shmem/tmpfs that for long time intentionally mapped
> > > > with PMDs across i_size.
> > > > 
> > > > Link: https://lkml.kernel.org/r/20251027115636.82382-3-kirill@shutemov.name
> > > > Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")
> > > > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > > Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > > Cc: Christian Brauner <brauner@kernel.org>
> > > > Cc: "Darrick J. Wong" <djwong@kernel.org>
> > > > Cc: Dave Chinner <david@fromorbit.com>
> > > > Cc: David Hildenbrand <david@redhat.com>
> > > > Cc: Hugh Dickins <hughd@google.com>
> > > > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > > > Cc: Liam Howlett <liam.howlett@oracle.com>
> > > > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > > > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > > Cc: Michal Hocko <mhocko@suse.com>
> > > > Cc: Mike Rapoport <rppt@kernel.org>
> > > > Cc: Rik van Riel <riel@surriel.com>
> > > > Cc: Shakeel Butt <shakeel.butt@linux.dev>
> > > > Cc: Suren Baghdasaryan <surenb@google.com>
> > > > Cc: Vlastimil Babka <vbabka@suse.cz>
> > > > Cc: <stable@vger.kernel.org>
> > > > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > > > (cherry picked from commit fa04f5b60fda62c98a53a60de3a1e763f11feb41)
> > > > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > > > ---
> > > 
> > > Does not apply to 6.17.y at all :(
> > > 
> > > Did you forget to apply this on top of other commits?
> > 
> > Hm. It applies cleanly on v6.17.8:
> > 
> > ❯ git log -1 --oneline @
> > 8ac42a63c561 (HEAD) Linux 6.17.8
> > ❯ b4 shazam 20251120165221.892852-1-kas@kernel.org
> > Grabbing thread from lore.kernel.org/all/20251120165221.892852-1-kas@kernel.org/t.mbox.gz
> > Breaking thread to remove parents of 20251120165221.892852-1-kas@kernel.org
> > Checking for newer revisions
> > Grabbing search results from lore.kernel.org
> > Analyzing 2 messages in the thread
> > Analyzing 1 code-review messages
> > Checking attestation on all messages, may take a moment...
> > ---
> >   ✓ [PATCH] mm/truncate: unmap large folio on split failure
> >   ---
> >   ✓ Signed: DKIM/kernel.org
> > ---
> > Total patches: 1
> > ---
> > Applying: mm/truncate: unmap large folio on split failure
> > 
> > Do you have anything on top of v6.17.8 in your 6.17.y queue?
> > 
> > My other backport to 6.17.y doesn't interfere with the patch either.
> 
> I see 6.17.9-rc1 includes
> 
> 	53241caf24c7 ("mm/huge_memory: do not change split_huge_page*() target order silently")
> 
> With the patch applied, fa04f5b60fda ("mm/truncate: unmap large folio on
> split failure") can be cherry-picked cleanly.

Ah, that worked, thanks!

greg k-h