Re: [PATCH v3 02/18] mm/huge_memory: fix rejection of swap cache folios with a mapping

[Kiryl Shutsemau]

On Thu, Aug 27, 2026 at 06:00:33PM +0200, David Hildenbrand (Arm) wrote:
> On 8/20/26 20:55, Kairui Song via B4 Relay wrote:
> > From: Kairui Song <kasong@xxxxxxxxxxx>
> > 
> > A folio in the swap cache cannot be split if it has a mapping (shmem).
> > The split code does a defensive check for this in
> > __folio_freeze_and_split_unmapped, after the folio ref has been frozen
> > and the NR_SHMEM_THPS/NR_FILE_THPS counters have been decremented. It
> > rejects the split and returns -EINVAL without unfreezing the folio or
> > restoring the counters. That error path is buggy: if it is ever taken,
> > it leaves the folio frozen and stuck, skews the counters, and fires
> > the VM_WARN_ON_ONCE_FOLIO for a state that is actually legitimate.
> > 
> > Check for this case up front in folio_check_splittable and return
> > -EBUSY before any state is modified, so the split routine always backs
> > out cleanly.
> > 
> > Also fix a bracket style issue that checkpatch.pl keeps complaining
> > about.
> > 
> > Fixes: 00527733d0dc ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
> > Fixes: 714b056c8321 ("mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio_split")
> > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> > ---
> >  mm/huge_memory.c | 27 ++++++++++++++++-----------
> >  1 file changed, 16 insertions(+), 11 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index ced400f72d43..a6759a14e057 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3878,6 +3878,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
> >  int folio_check_splittable(struct folio *folio, unsigned int new_order,
> >  			   enum split_type split_type)
> >  {
> > +	bool is_anon = folio_test_anon(folio);
> > +	bool is_swapcache = folio_test_swapcache(folio);
> 
> Both const please.

I see a lot of const everywhere in mm code now. I feel I missed the memo.
Do they make a difference? I was relying on "Compiler does its
job"(TM) for things like this before.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov