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

[Brian Foster <bfoster@redhat.com>]

On Thu, Jun 07, 2018 at 03:21:32PM +1000, 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>
> ---

Looks fine modulo the header thing Allison already pointed out:

Reviewed-by: Brian Foster <bfoster@redhat.com>

BTW, shouldn't we also be using XFS_TRANS_NOFS in
xfs_iomap_write_allocate()?

Brian

>  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);
>  	}
>  
>  	/* 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>
> -- 
> 2.17.0
> 
> --
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