xfs: handle negative wbc->nr_to_write during sync writeback
During data integrity (WB_SYNC_ALL) writeback, wbc->nr_to_write will go negative on inodes with more than 1024 dirty pages due to implementation details of write_cache_pages(). Currently XFS will abort page clustering in writeback once nr_to_write drops below zero, and so for data integrity writeback we will do very inefficient page at a time allocation and IO submission for inodes with large numbers of dirty pages. Fix this by only aborting the page clustering code when wbc->nr_to_write is negative and the sync mode is WB_SYNC_NONE. Cc: <stable@kernel.org> Signed-off-by: NDave Chinner <dchinner@redhat.com> Reviewed-by: NChristoph Hellwig <hch@lst.de>
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