xfs: don't serialise direct IO reads on page cache checks (0c38a251) · Commits · 戴 / test

There is no need to grab the i_mutex of the IO lock in exclusive mode if we don't need to invalidate the page cache. Taking these locks on every direct IO effective serialises them as taking the IO lock in exclusive mode has to wait for all shared holders to drop the lock. That only happens when IO is complete, so effective it prevents dispatch of concurrent direct IO reads to the same inode. Fix this by taking the IO lock shared to check the page cache state, and only then drop it and take the IO lock exclusively if there is work to be done. Hence for the normal direct IO case, no exclusive locking will occur. Signed-off-by:

Dave Chinner <dchinner@redhat.com> Tested-by:

Joern Engel <joern@logfs.org> Reviewed-by:

Christoph Hellwig <hch@lst.de> Signed-off-by:

Alex Elder <aelder@sgi.com>

fs/xfs/xfs_file.c

+14 −3

Original line number	Diff line number	Diff line
		@@ -317,7 +317,19 @@ xfs_file_aio_read(
		if (XFS_FORCED_SHUTDOWN(mp))
		return -EIO;

		if (unlikely(ioflags & IO_ISDIRECT)) {
		/*
		* Locking is a bit tricky here. If we take an exclusive lock
		* for direct IO, we effectively serialise all new concurrent
		* read IO to this file and block it behind IO that is currently in
		* progress because IO in progress holds the IO lock shared. We only
		* need to hold the lock exclusive to blow away the page cache, so
		* only take lock exclusively if the page cache needs invalidation.
		* This allows the normal direct IO case of no page cache pages to
		* proceeed concurrently without serialisation.
		*/
		xfs_rw_ilock(ip, XFS_IOLOCK_SHARED);
		if ((ioflags & IO_ISDIRECT) && inode->i_mapping->nrpages) {
		xfs_rw_iunlock(ip, XFS_IOLOCK_SHARED);
		xfs_rw_ilock(ip, XFS_IOLOCK_EXCL);

		if (inode->i_mapping->nrpages) {
		@@ -330,8 +342,7 @@ xfs_file_aio_read(
		}
		}
		xfs_rw_ilock_demote(ip, XFS_IOLOCK_EXCL);
		} else
		xfs_rw_ilock(ip, XFS_IOLOCK_SHARED);
		}

		trace_xfs_file_read(ip, size, iocb->ki_pos, ioflags);

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