linux-kernel - Re: Endless calls to xas_split_alloc() due to corrupted xarray entry

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Message-ID: <CAHk-=who82OKiXyTiCG3rUaiicO_OB9prVvZQBzg6GDGhdp+Ew@mail.gmail.com>
Date: Wed, 19 Jun 2024 08:48:28 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: David Hildenbrand <david@...hat.com>, Gavin Shan <gshan@...hat.com>, 
	Bagas Sanjaya <bagasdotme@...il.com>, Zhenyu Zhang <zhenyzha@...hat.com>, 
	Linux XFS <linux-xfs@...r.kernel.org>, 
	Linux Filesystems Development <linux-fsdevel@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Shaoqin Huang <shahuang@...hat.com>, 
	Chandan Babu R <chandan.babu@...cle.com>, "Darrick J. Wong" <djwong@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Endless calls to xas_split_alloc() due to corrupted xarray entry

On Wed, 19 Jun 2024 at 07:31, Matthew Wilcox <willy@...radead.org> wrote:
>
> Actually, it's 11.  We can't split an order-12 folio because we'd have
> to allocate two levels of radix tree, and I decided that was too much
> work.  Also, I didn't know that ARM used order-13 PMD size at the time.
>
> I think this is the best fix (modulo s/12/11/).

Can we use some more descriptive thing than the magic constant 11 that
is clearly very subtle.

Is it "XA_CHUNK_SHIFT * 2 - 1"

IOW, something like

   #define MAX_XAS_ORDER (XA_CHUNK_SHIFT * 2 - 1)
   #define MAX_PAGECACHE_ORDER min(HPAGE_PMD_ORDER,12)

except for the non-TRANSPARENT_HUGEPAGE case where it currently does

  #define MAX_PAGECACHE_ORDER    8

and I assume that "8" is just "random round value, smaller than 11"?

             Linus