Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

[Balbir Singh]

On Thu, Jun 11, 2026 at 10:49:59AM +1000, Alistair Popple wrote:
> On 2026-06-06 at 02:20 +1000, Zi Yan <ziy@xxxxxxxxxx> wrote...
> > On 5 Jun 2026, at 5:22, David Hildenbrand (Arm) wrote:
> > 
> > > On 6/5/26 07:17, Alistair Popple wrote:
> > >> On 2026-05-15 at 10:09 +1000, Balbir Singh <balbirs@xxxxxxxxxx> wrote...
> > >>> On 4/9/26 17:52, David Hildenbrand (Arm) wrote:
> > >>>>
> > >>>> Hm, what if secretmem folio just got truncated? I hate to rely on some
> > >>>> handling in the caller to detect truncation differently during GUP-fast,
> > >>>> but this function returning "true".
> > >>>>
> > >>>
> > >>> Can secretmem folios be truncated? I assume you are referring to
> > >>> ftruncate(), I am looking at the setattr implementation of secretmem
> > >>> and it does not seem like it can be truncated.
> > >>>
> > >>>> Zi is working on a way to distinguish folios from non-folio things: that
> > >>>> we can identify whatever was added through vm_insert_page().
> > >>>>
> > >>>> Because that's really the key problem here: vm_insert_page() pages are
> > >>>> not actually folios, they just look like a folio today, but looking at
> > >>>> fields like ->mapping does not make any sense.
> > >>>>
> > >>>
> > >>> I still think this is a short term fix worth having until we get
> > >>> Zi's fixes
> > >>
> > >> Agreed - this clearly results in a performance regression and we have customers
> > >> reporting it as such. I think Zi might be out of the office atm but when I last
> > >> spoke to him he agreed this fix should go in for now and his approach will come
> > >> later.
> > >
> > > He should be back now. But yeah, let's get this fix in.
> > 
> > Right, I will work on my solution soon. Meanwhile, this change
> > is simpler and can be easily back ported.
> 
> Thanks.
> 
> > BTW, should it be back ported to older kernels?
> 
> Yes to any with f002882ca369 ("mm: merge folio_is_secretmem() and
> folio_fast_pin_allowed() into gup_fast_folio_allowed()") - we have got bug
> reports as a result of the lowered perf that commit causes.
> 
>  - Alistair
> 

Ping @akpm to pick this up, if there is no objection @david

Balbir