linux-kernel - Re: [PATCH] mm/shmem: fix uninitialized folio in shmem

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Message-ID: <aV1Yb4r9_ukWoc8i@casper.infradead.org>
Date: Tue, 6 Jan 2026 18:46:07 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Brian Foster <bfoster@...hat.com>, Barry Song <21cnbao@...il.com>,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	syzbot+178fff6149127421c2cc@...kaller.appspotmail.com
Subject: Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink

On Tue, Jan 06, 2026 at 11:47:44AM +0800, Baolin Wang wrote:
> Why I prefer Barry's fix: First, the symlink folio is marked Uptodate after
> copying the symlink name, but the whole folio hasn’t been initialized, which
> seems unreasonable to me. Second, as I said before, using the 'PG_owner_2'
> flag to mark this uncommon case doesn’t seem worthwhile. Currently, IIUC the
> 'PG_owner_2' is only used by btrfs; if we ever want to remove the
> 'PG_owner_2', this uncommon symlink case shouldn’t block its removal.

PG_owner_2 is aliased with PG_mappedtodisk [1], so it's used by every
filesystem which uses buffer_heads (whether mentioned in that filesystem
or not).  btrfs was switched from using private_2 to using owner_2
a little over a year ago.  PG_owner_2 is not on the list of flags to be
removed; that's PG_private, PG_private_2 and PG_reserved.

[1] include/linux/page-flags.h:     PG_mappedtodisk = PG_owner_2,