Re: [PATCH 2/2] xfs: xfs_reflink_convert_cow() memory allocation deadlock

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jun 06, 2018 at 10:56:50PM -0700, Allison Henderson wrote:
> On 06/06/2018 10:21 PM, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > xfs_reflink_convert_cow() manipulates the incore extent list
> > in GFP_KERNEL context in the IO submission path whilst holding
> > locked pages under writeback. This is a memory reclaim deadlock
> > vector. This code is not in a transaction, so any memory allocations
> > it makes aren't protected via the memalloc_nofs_save() context that
> > transactions carry.
> > 
> > Hence we need to run this call under memalloc_nofs_save() context to
> > prevent potential memory allocations from being run as GFP_KERNEL
> > and deadlocking.
> > 
> > Signed-Off-By: Dave Chinner <dchinner@redhat.com>
> > ---
> >   fs/xfs/xfs_aops.c  | 11 +++++++++++
> >   fs/xfs/xfs_buf.c   |  1 -
> >   fs/xfs/xfs_linux.h |  1 +
> >   3 files changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 767d53222f31..1eb625fdcb1e 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -531,8 +531,19 @@ xfs_submit_ioend(
> >   {
> >   	/* Convert CoW extents to regular */
> >   	if (!status && ioend->io_type == XFS_IO_COW) {
> > +		/*
> > +		 * Yuk. This can do memory allocation, but is not a
> > +		 * transactional operation so everything is done in GFP_KERNEL
> > +		 * context. That can deadlock, because we hold pages in
> > +		 * writeback state and GFP_KERNEL allocations can block on them.
> > +		 * Hence we must operate in nofs conditions here.
> > +		 */
> > +		unsigned nofs_flag;
> > +
> > +		nofs_flag = memalloc_nofs_save();
> >   		status = xfs_reflink_convert_cow(XFS_I(ioend->io_inode),
> >   				ioend->io_offset, ioend->io_size);
> > +		memalloc_nofs_restore(nofs_flag);

DOH. :)

> >   	}
> >   	/* Reserve log space if we might write beyond the on-disk inode size. */
> > diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> > index 980bc48979e9..e9c058e3761c 100644
> > --- a/fs/xfs/xfs_buf.c
> > +++ b/fs/xfs/xfs_buf.c
> > @@ -21,7 +21,6 @@
> >   #include <linux/migrate.h>
> >   #include <linux/backing-dev.h>
> >   #include <linux/freezer.h>
> > -#include <linux/sched/mm.h>
> >   #include "xfs_format.h"
> >   #include "xfs_log_format.h"
> > diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
> > index ae1e66fa3f61..1631cf4546f2 100644
> > --- a/fs/xfs/xfs_linux.h
> > +++ b/fs/xfs/xfs_linux.h
> > @@ -26,6 +26,7 @@ typedef __u32			xfs_nlink_t;
> >   #include <linux/semaphore.h>
> >   #include <linux/mm.h>
> > +#include <linux/sched/mm.h>
> >   #include <linux/kernel.h>
> >   #include <linux/blkdev.h>
> >   #include <linux/slab.h>
> > 
> Looks, ok.  Was moving the header include intentional?  Just clean up maybe?
> Other than that, looks good.

I can't speak for Dave, but I'll point out that memalloc_nofs_restore is
declared in linux/sched/mm.h, so the #include hoist makes the symbol
available to the aops code in such a manner that now it's available to
all the xfs code so that we don't have to remember this...

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> 
> Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
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