Re: [linux-next:master] [mm] 0fa2857d23: WARNING:at_mm/page_alloc.c:#__alloc_pages_noprof

[Yosry Ahmed]

On Mon, Jun 24, 2024 at 12:26 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jun 24, 2024 at 11:57:45AM -0700, Yosry Ahmed wrote:
> > On Mon, Jun 24, 2024 at 11:56 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Jun 24, 2024 at 11:53:30AM -0700, Yosry Ahmed wrote:
> > > > After a page is swapped out during reclaim, __remove_mapping() will
> > > > call __delete_from_swap_cache() to replace the swap cache entry with a
> > > > shadow entry (which is an xa_value).
> > >
> > > Special entries are disjoint from shadow entries.  Shadow entries have
> > > the last two bits as 01 or 11 (are congruent to 1 or 3 modulo 4).
> > > Special entries have values below 4096 which end in 10 (are congruent
> > > to 2 modulo 4).
> >
> > You are implying that we would no longer have a shadow entry for such
> > zero folios, because we will be storing a special entry instead.
> > Right?
>
> umm ... maybe I have a misunderstanding here.
>
> I'm saying that there wouldn't be a _swap_ entry here because the folio
> wouldn't be stored anywhere on the swap device.  But there could be a
> _shadow_ entry.  Although if the page is full of zeroes, it was probably
> never referenced and doesn't really need a shadow entry.

Is it possible to have a shadow entry AND a special entry (e.g.
XA_ZERO_ENTRY) at the same index? This is what would be required to
maintain the current behavior (assuming we really need the shadow
entries for such zeroed folios).