Re: [PATCH v2] xfs: clear PF_MEMALLOC before exiting xfsaild thread

Darrick J. Wong
2020-03-09T18:19:19Z

On Mon, Mar 09, 2020 at 11:04:52AM -0700, Eric Biggers wrote:
> On Mon, Mar 09, 2020 at 09:24:39AM -0700, Darrick J. Wong wrote:
> > On Sun, Mar 08, 2020 at 09:34:30PM -0700, Eric Biggers wrote:
> > > 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() ends 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.
> > >
> > > Fix this in xfsaild() by using the helper functions to save and restore
> > > PF_MEMALLOC.
> > >
> > > This can be reproduced as follows in the kvm-xfstests test appliance
> > > modified to add the 'acct' Debian package, and with kvm-xfstests's
> > > recommended kconfig modified to add CONFIG_BSD_PROCESS_ACCT=y:
> > >
> > > mkfs.ext2 -F /dev/vdb
> > > mount /vdb -t ext4
> > > touch /vdb/file
> > > chattr +S /vdb/file
> >
> > Does this trip if the process accounting file is also on an xfs
> > filesystem?
> >
> > > accton /vdb/file
> > > mkfs.xfs -f /dev/vdc
> > > mount /vdc
> > > umount /vdc
> >
> > ...and if so, can this be turned into an fstests case, please?
>
> I wasn't expecting it, but it turns out it does actually trip a similar warning
> in iomap_do_writepage():
>
> mkfs.xfs -f /dev/vdb
> mount /vdb
> touch /vdb/file
> chattr +S /vdb/file
> accton /vdb/file
> mkfs.xfs -f /dev/vdc
> mount /vdc
> umount /vdc
>
> causes...
>
> WARNING: CPU: 1 PID: 336 at fs/iomap/buffered-io.c:1534
> CPU: 1 PID: 336 Comm: xfsaild/vdc Not tainted 5.6.0-rc5 #3
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
> RIP: 0010:iomap_do_writepage+0x16b/0x1f0 fs/iomap/buffered-io.c:1534
> [...]
> Call Trace:
> write_cache_pages+0x189/0x4d0 mm/page-writeback.c:2238
> iomap_writepages+0x1c/0x33 fs/iomap/buffered-io.c:1642
> xfs_vm_writepages+0x65/0x90 fs/xfs/xfs_aops.c:578
> do_writepages+0x41/0xe0 mm/page-writeback.c:2344
> __filemap_fdatawrite_range+0xd2/0x120 mm/filemap.c:421
> file_write_and_wait_range+0x71/0xc0 mm/filemap.c:760
> xfs_file_fsync+0x7a/0x2b0 fs/xfs/xfs_file.c:114
> generic_write_sync include/linux/fs.h:2867 [inline]
> xfs_file_buffered_aio_write+0x379/0x3b0 fs/xfs/xfs_file.c:691
> call_write_iter include/linux/fs.h:1901 [inline]
> new_sync_write+0x130/0x1d0 fs/read_write.c:483
> __kernel_write+0x54/0xe0 fs/read_write.c:515
> do_acct_process+0x122/0x170 kernel/acct.c:522
> slow_acct_process kernel/acct.c:581 [inline]
> acct_process+0x1d4/0x27c kernel/acct.c:607
> do_exit+0x83d/0xbc0 kernel/exit.c:791
> kthread+0xf1/0x140 kernel/kthread.c:257
> ret_from_fork+0x27/0x50 arch/x86/entry/entry_64.S:352
>
> So sure, since it's not necessarily a multi-filesystem thing, I can try to turn
> it into an xfstest. There's currently no way to enable BSD process accounting
> in xfstests though, so we'll either need to make the test depend on the 'acct'
> program or add a helper test program.
>
> Also, do you want me to update the commit message again, to mention the above
> case?

I think it's worth mentioning that this is a general problem that
applies any time the process accounting file has that sync flag set,
since this problem isn't specific to ext4 + xfs.

(and now I wonder how many other places in the kernel suffer from these
kinds of file write surprises...)

--D

> - Eric