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cifs: clear PF_MEMALLOC before exiting demultiplex thread

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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>
Signed-off-by: Steve French <stfrench@microsoft.com>
This commit is contained in:
Eric Biggers 2020-03-08 22:58:20 -07:00 committed by Steve French
parent 266b9fecc5
commit dc920277f1
1 changed files with 4 additions and 1 deletions
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5
fs/cifs/connect.c
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@ -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>
@ -1114,8 +1115,9 @@ 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);
@ -1269,6 +1271,7 @@ next_pdu:
set_current_state(TASK_RUNNING);
}
memalloc_noreclaim_restore(noreclaim_flag);
module_put_and_exit(0);
}