linux-kernel - [PATCH 0/2] Improve dmesg output for swapfile+hibernation

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Date: Wed, 22 May 2024 16:46:56 +0900
From: Sukrit Bhatnagar <Sukrit.Bhatnagar@...y.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
	Pavel Machek <pavel@....cz>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-xfs@...r.kernel.org,
	linux-pm@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Sukrit.Bhatnagar@...y.com
Subject: [PATCH 0/2] Improve dmesg output for swapfile+hibernation

While trying to use a swapfile for hibernation, I noticed that the suspend
process was failing when it tried to search for the swap to use for snapshot.
I had created the swapfile on ext4 and got the starting physical block offset
using the filefrag command.

Upon looking at the swap activation code, I realized that the iomap part is
doing some rounding up for the physical offset of the swapfile.
Then I checked the block size of the filesystem, which was actually set to
1KB by default in my environment.
(This was in buildroot, using the genimage utility to create the VM disk
partitions, filesystems etc.)

The block offset is rounded-up and stored in the swap extents metadata by
iomap code, but as the exact value is lost for 1KB-block filesystem, hibernate
cannot read back the swap header after it finishes writing data pages to swap.

Note that this is not a bug in my understanding. Both swapfile and hibernate
subsystems have the correct handling of this edge case, individually.

Another observation was that we need to rely on external commands, such as
filefrag for getting the swapfile offset value. This value can be conveniently
printed in dmesg output when doing swapon.

Sukrit Bhatnagar (2):
  iomap: swap: print warning for unaligned swapfile
  mm: swap: print starting physical block offset in swapon

 fs/iomap/swapfile.c | 10 ++++++++++
 mm/swapfile.c       |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

-- 
2.34.1