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[v3] cifs: clear PF_MEMALLOC before exiting demultiplex thread

Message ID 20200309055820.272703-1-ebiggers@kernel.org
State New
Headers show
Series [v3] cifs: clear PF_MEMALLOC before exiting demultiplex thread | expand

Commit Message

Eric Biggers March 9, 2020, 5:58 a.m. UTC
From: Eric Biggers <ebiggers@google.com>

Leaving PF_MEMALLOC set when exiting a kthread causes it to remain set
during do_exit().  That can confuse things.  For example, if BSD process
accounting is enabled and the accounting file has FS_SYNC_FL set and is
located on an ext4 filesystem without a journal, then do_exit() can end
up calling ext4_write_inode().  That triggers the
WARN_ON_ONCE(current->flags & PF_MEMALLOC) there, as it assumes
(appropriately) that inodes aren't written when allocating memory.

This was originally reported for another kernel thread, xfsaild() [1].
cifs_demultiplex_thread() also exits with PF_MEMALLOC set, so it's
potentially subject to this same class of issue -- though I haven't been
able to reproduce the WARN_ON_ONCE() via CIFS, since unlike xfsaild(),
cifs_demultiplex_thread() is sent SIGKILL before exiting, and that
interrupts the write to the BSD process accounting file.

Either way, leaving PF_MEMALLOC set is potentially problematic.  Let's
clean this up by properly saving and restoring PF_MEMALLOC.

[1] https://lore.kernel.org/r/0000000000000e7156059f751d7b@google.com

Signed-off-by: Eric Biggers <ebiggers@google.com>

v3: improved commit message
v2: added missing include

 fs/cifs/connect.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
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diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 4804d1df8c1cf..97b8eb585cf9d 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -21,6 +21,7 @@ 
 #include <linux/fs.h>
 #include <linux/net.h>
 #include <linux/string.h>
+#include <linux/sched/mm.h>
 #include <linux/sched/signal.h>
 #include <linux/list.h>
 #include <linux/wait.h>
@@ -1164,8 +1165,9 @@  cifs_demultiplex_thread(void *p)
 	struct task_struct *task_to_wake = NULL;
 	struct mid_q_entry *mids[MAX_COMPOUND];
 	char *bufs[MAX_COMPOUND];
+	unsigned int noreclaim_flag;
-	current->flags |= PF_MEMALLOC;
+	noreclaim_flag = memalloc_noreclaim_save();
 	cifs_dbg(FYI, "Demultiplex PID: %d\n", task_pid_nr(current));
 	length = atomic_inc_return(&tcpSesAllocCount);
@@ -1320,6 +1322,7 @@  cifs_demultiplex_thread(void *p)
+	memalloc_noreclaim_restore(noreclaim_flag);