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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jun 24, 2024 at 08:50:45PM GMT, Matthew Wilcox wrote:
> On Mon, Jun 24, 2024 at 12:34:04PM -0700, Yosry Ahmed wrote:
> > On Mon, Jun 24, 2024 at 12:26 PM Matthew Wilcox <willy@infradead.org> 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@infradead.org> 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).
> 
> No, just like it's not possible to have a swap entry and a shadow entry
> at the same location.  You have to choose.  But the zero entry is an
> alternative to the swap entry, not the shadow entry.
> 
> As I understand the swap cache, at the moment, you can have four
> possible results from a lookup:
> 
>  - NULL
>  - a swap entry
>  - a shadow entry
>  - a folio
> 
> Do I have that wrong?

I don't think we have swap entry in the swapcache (underlying xarray).
The swap entry is used as an index to find the folio or shadow entry.