+ shmem-remove-check-for-folio-lock-on-writepage.patch added to mm-unstable branch

+ shmem-remove-check-for-folio-lock-on-writepage.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: shmem: remove check for folio lock on writepage()
has been added to the -mm mm-unstable branch.  Its filename is
     shmem-remove-check-for-folio-lock-on-writepage.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/shmem-remove-check-for-folio-lock-on-writepage.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Luis Chamberlain <mcgrof@kernel.org>
Subject: shmem: remove check for folio lock on writepage()
Date: Thu, 9 Mar 2023 15:05:40 -0800

Patch series "tmpfs: add the option to disable swap", v2.

I'm doing this work as part of future experimentation with tmpfs and the
page cache, but given a common complaint found about tmpfs is the
innability to work without the page cache I figured this might be useful
to others.  It turns out it is -- at least Christian Brauner indicates
systemd uses ramfs for a few use-cases because they don't want to use swap
and so having this option would let them move over to using tmpfs for
those small use cases, see systemd-creds(1).

To see if you hit swap:

mkswap /dev/nvme2n1
swapon /dev/nvme2n1
free -h

With swap - what we see today
=============================
mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       2.6Gi       1.2Gi       2.2Gi       2.2Gi       1.2Gi
Swap:           99Gi       2.8Gi        97Gi


Without swap
=============

free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       387Mi       3.4Gi       2.1Mi        57Mi       3.3Gi
Swap:           99Gi          0B        99Gi
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
dd if=/dev/urandom of=/data-tmpfs/5g-rand2 bs=1G count=5
free -h
               total        used        free      shared  buff/cache   available
Mem:           3.7Gi       2.6Gi       1.2Gi       2.3Gi       2.3Gi       1.1Gi
Swap:           99Gi        21Mi        99Gi

The mix and match remount testing
=================================

# Cannot disable swap after it was first enabled:
mount -t tmpfs            -o size=5G           tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
mount: /data-tmpfs: mount point not mounted or bad option.
       dmesg(1) may have more information after failed mount system call.
dmesg -c
tmpfs: Cannot disable swap on remount

# Remount with the same noswap option is OK:
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G -o noswap tmpfs /data-tmpfs/
dmesg -c

# Trying to enable swap with a remount after it first disabled:
mount -t tmpfs            -o size=5G -o noswap tmpfs /data-tmpfs/
mount -t tmpfs -o remount -o size=5G           tmpfs /data-tmpfs/
mount: /data-tmpfs: mount point not mounted or bad option.
       dmesg(1) may have more information after failed mount system call.
dmesg -c
tmpfs: Cannot enable swap on remount if it was disabled on first mount


This patch (of 6):

Matthew notes we should not need to check the folio lock on the
writepage() callback so remove it.  This sanity check has been lingering
since linux-history days.  We remove this as we tidy up the writepage()
callback to make things a bit clearer.

Link: https://lkml.kernel.org/r/20230309230545.2930737-1-mcgrof@kernel.org
Link: https://lkml.kernel.org/r/20230309230545.2930737-2-mcgrof@kernel.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Cc: Adam Manzanares <a.manzanares@samsung.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
---


--- a/mm/shmem.c~shmem-remove-check-for-folio-lock-on-writepage
+++ a/mm/shmem.c
@@ -1336,7 +1336,6 @@ static int shmem_writepage(struct page *
 		folio_clear_dirty(folio);
 	}
 
-	BUG_ON(!folio_test_locked(folio));
 	mapping = folio->mapping;
 	index = folio->index;
 	inode = mapping->host;
_

Patches currently in -mm which might be from mcgrof@kernel.org are

shmem-remove-check-for-folio-lock-on-writepage.patch
shmem-set-shmem_writepage-variables-early.patch
shmem-move-reclaim-check-early-on-writepages.patch
shmem-skip-page-split-if-were-not-reclaiming.patch
shmem-update-documentation.patch
shmem-add-support-to-ignore-swap.patch