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 20:26:47 +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 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.