commit 3deaa7190a8da38453c4fabd9dec7f66d17fff67
Author: Shaohua Li <shaohua.li@intel.com>
Date:   Fri Feb 3 15:37:17 2012 -0800

    readahead: fix pipeline break caused by block plug
    
    Herbert Poetzl reported a performance regression since 2.6.39.  The test
    is a simple dd read, but with big block size.  The reason is:
    
    T1: ra (A, A+128k), (A+128k, A+256k)
    T2: lock_page for page A, submit the 256k
    T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
    because of plug and there isn't any lock_page till we hit page A+256k
    because all pages from A to A+256k is in memory
    T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
    submitted again.
    T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is
    waitting for (A+256k, A+512k) finish.
    
    There is no request to disk in T3 and T4, so readahead pipeline breaks.
    
    We really don't need block plug for generic_file_aio_read() for buffered
    I/O.  The readahead already has plug and has fine grained control when I/O
    should be submitted.  Deleting plug for buffered I/O fixes the regression.
    
    One side effect is plug makes the request size 256k, the size is 128k
    without it.  This is because default ra size is 128k and not a reason we
    need plug here.
    
    Vivek said:
    
    : We submit some readahead IO to device request queue but because of nested
    : plug, queue never gets unplugged.  When read logic reaches a page which is
    : not in page cache, it waits for page to be read from the disk
    : (lock_page_killable()) and that time we flush the plug list.
    :
    : So effectively read ahead logic is kind of broken in parts because of
    : nested plugging.  Removing top level plug (generic_file_aio_read()) for
    : buffered reads, will allow unplugging queue earlier for readahead.
    
    Signed-off-by: Shaohua Li <shaohua.li@intel.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    Reported-by: Herbert Poetzl <herbert@13thfloor.at>
    Tested-by: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Vivek Goyal <vgoyal@redhat.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index 97f49ed..b662757 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 	unsigned long seg = 0;
 	size_t count;
 	loff_t *ppos = &iocb->ki_pos;
-	struct blk_plug plug;
 
 	count = 0;
 	retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
 	if (retval)
 		return retval;
 
-	blk_start_plug(&plug);
-
 	/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
 	if (filp->f_flags & O_DIRECT) {
 		loff_t size;
@@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 			retval = filemap_write_and_wait_range(mapping, pos,
 					pos + iov_length(iov, nr_segs) - 1);
 			if (!retval) {
+				struct blk_plug plug;
+
+				blk_start_plug(&plug);
 				retval = mapping->a_ops->direct_IO(READ, iocb,
 							iov, pos, nr_segs);
+				blk_finish_plug(&plug);
 			}
 			if (retval > 0) {
 				*ppos = pos + retval;
@@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 			break;
 	}
 out:
-	blk_finish_plug(&plug);
 	return retval;
 }
 EXPORT_SYMBOL(generic_file_aio_read);