linux-kernel - [PATCH 4.19 125/125] cgroup,writeback: dont switch wbs immediately on dead wbs if the memcg is dead

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 11 Nov 2019 19:29:24 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 4.19 125/125] cgroup,writeback: dont switch wbs immediately on dead wbs if the memcg is dead

From: Tejun Heo <tj@...nel.org>

commit 65de03e251382306a4575b1779c57c87889eee49 upstream.

cgroup writeback tries to refresh the associated wb immediately if the
current wb is dead.  This is to avoid keeping issuing IOs on the stale
wb after memcg - blkcg association has changed (ie. when blkcg got
disabled / enabled higher up in the hierarchy).

Unfortunately, the logic gets triggered spuriously on inodes which are
associated with dead cgroups.  When the logic is triggered on dead
cgroups, the attempt fails only after doing quite a bit of work
allocating and initializing a new wb.

While c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if mapping
has no dirty pages") alleviated the issue significantly as it now only
triggers when the inode has dirty pages.  However, the condition can
still be triggered before the inode is switched to a different cgroup
and the logic simply doesn't make sense.

Skip the immediate switching if the associated memcg is dying.

This is a simplified version of the following two patches:

 * https://lore.kernel.org/linux-mm/20190513183053.GA73423@dennisz-mbp/
 * http://lkml.kernel.org/r/156355839560.2063.5265687291430814589.stgit@buzz

Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Fixes: e8a7abf5a5bd ("writeback: disassociate inodes from dying bdi_writebacks")
Acked-by: Dennis Zhou <dennis@...nel.org>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/fs-writeback.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -582,10 +582,13 @@ void wbc_attach_and_unlock_inode(struct
 	spin_unlock(&inode->i_lock);

 	/*
-	 * A dying wb indicates that the memcg-blkcg mapping has changed
-	 * and a new wb is already serving the memcg.  Switch immediately.
+	 * A dying wb indicates that either the blkcg associated with the
+	 * memcg changed or the associated memcg is dying.  In the first
+	 * case, a replacement wb should already be available and we should
+	 * refresh the wb immediately.  In the second case, trying to
+	 * refresh will keep failing.
 	 */
-	if (unlikely(wb_dying(wbc->wb)))
+	if (unlikely(wb_dying(wbc->wb) && !css_is_dying(wbc->wb->memcg_css)))
 		inode_switch_wbs(inode, wbc->wb_id);
 }