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

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Date: Mon, 24 Jun 2024 20:50:45 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: kernel test robot <oliver.sang@...el.com>,
	Usama Arif <usamaarif642@...il.com>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, Linux Memory Management List <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Nhat Pham <nphamcs@...il.com>, David Hildenbrand <david@...hat.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: [linux-next:master] [mm] 0fa2857d23:
 WARNING:at_mm/page_alloc.c:#__alloc_pages_noprof

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@...radead.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@...radead.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?