On Wed, Mar 15, 2017 at 02:24:14PM +0200, Nikolay Borisov wrote:
> From: Brian Foster <bfoster@xxxxxxxxxx>
>
> The total field from struct xfs_alloc_arg is a bit of an unknown
> commodity. It is documented as the total block requirement for the
> transaction and is used in this manner from most call sites by virtue of
> passing the total block reservation of the transaction associated with
> an allocation. Several xfs_bmapi_write() callers pass hardcoded values
> of 0 or 1 for the total block requirement, which is a historical oddity
> without any clear reasoning.
>
> The xfs_iomap_write_direct() caller, for example, passes 0 for the total
> block requirement. This has been determined to cause problems in the
> form of ABBA deadlocks of AGF buffers due to incorrect AG selection in
> the block allocator. Specifically, the xfs_alloc_space_available()
> function incorrectly selects an AG that doesn't actually have sufficient
> space for the allocation. This occurs because the args.total field is 0
> and thus the remaining free space check on the AG doesn't actually
> consider the size of the allocation request. This locks the AGF buffer,
> the allocation attempt proceeds and ultimately fails (in
> xfs_alloc_fix_minleft()), and xfs_alloc_vexent() moves on to the next
> AG. In turn, this can lead to incorrect AG locking order (if the
> allocator wraps around, attempting to lock AG 0 after acquiring AG N)
> and thus deadlock if racing with another operation. This problem has
> been reproduced via generic/299 on smallish (1GB) ramdisk test devices.
>
> To avoid this problem, replace the undocumented hardcoded total
> parameters from the iomap and utility callers to pass the block
> reservation used for the associated transaction. This is consistent with
> other xfs_bmapi_write() callers throughout XFS. The assumption is that
> the total field allows the selection of an AG that can handle the entire
> operation rather than simply the allocation/range being requested (e.g.,
> resulting btree splits, etc.). This addresses the aforementioned
> generic/299 hang by ensuring AG selection only occurs when the
> allocation can be satisfied by the AG.
>
> Reported-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Signed-off-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
> ---
>
> Here is the backport for 3.12
>
Seems fine to me, thanks. FWIW:
Acked-by: Brian Foster <bfoster@xxxxxxxxxx>
> fs/xfs/xfs_bmap_util.c | 2 +-
> fs/xfs/xfs_iomap.c | 8 ++++----
> 2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index 2843a0426bca..66172581dacb 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -1146,7 +1146,7 @@ retry:
> xfs_bmap_init(&free_list, &firstfsb);
> error = xfs_bmapi_write(tp, ip, startoffset_fsb,
> allocatesize_fsb, alloc_type, &firstfsb,
> - 0, imapp, &nimaps, &free_list);
> + resblks, imapp, &nimaps, &free_list);
> if (error) {
> goto error0;
> }
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 43e31b0b288b..da87415ac561 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -217,7 +217,7 @@ xfs_iomap_write_direct(
> xfs_bmap_init(&free_list, &firstfsb);
> nimaps = 1;
> error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb, bmapi_flag,
> - &firstfsb, 0, imap, &nimaps, &free_list);
> + &firstfsb, resblks, imap, &nimaps, &free_list);
> if (error)
> goto out_bmap_cancel;
>
> @@ -762,7 +762,7 @@ xfs_iomap_write_allocate(
> error = xfs_bmapi_write(tp, ip, map_start_fsb,
> count_fsb,
> XFS_BMAPI_STACK_SWITCH,
> - &first_block, 1,
> + &first_block, nres,
> imap, &nimaps, &free_list);
> if (error)
> goto trans_cancel;
> @@ -877,8 +877,8 @@ xfs_iomap_write_unwritten(
> xfs_bmap_init(&free_list, &firstfsb);
> nimaps = 1;
> error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
> - XFS_BMAPI_CONVERT, &firstfsb,
> - 1, &imap, &nimaps, &free_list);
> + XFS_BMAPI_CONVERT, &firstfsb, resblks,
> + &imap, &nimaps, &free_list);
> if (error)
> goto error_on_bmapi_transaction;
>
> --
> 2.7.4
>
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