[patch 1/6] mm: direct IO starvation improvement

[Nick Piggin <npiggin@suse.de>]

Direct IO can invalidate and sync a lot of pagecache pages in the mapping. A
4K direct IO will actually try to sync and/or invalidate the pagecache of the
entire file, for example (which might be many GB or TB large).

Improve this by doing range syncs. Also, memory no longer has to be unmapped
to catch the dirty bits for syncing, as dirty bits would remain coherent due to
dirty mmap accounting.

This fixes the immediate DM deadlocks when doing direct IO reads to block
device with a mounted filesystem, if only by papering over the problem somewhat
rather than addressing the fsync starvation cases.

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
 mm/filemap.c |   16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -1317,7 +1317,8 @@ generic_file_aio_read(struct kiocb *iocb
 			goto out; /* skip atime */
 		size = i_size_read(inode);
 		if (pos < size) {
-			retval = filemap_write_and_wait(mapping);
+			retval = filemap_write_and_wait_range(mapping, pos,
+					pos + iov_length(iov, nr_segs) - 1);
 			if (!retval) {
 				retval = mapping->a_ops->direct_IO(READ, iocb,
 							iov, pos, nr_segs);
@@ -2060,18 +2061,10 @@ generic_file_direct_write(struct kiocb *
 	if (count != ocount)
 		*nr_segs = iov_shorten((struct iovec *)iov, *nr_segs, count);
 
-	/*
-	 * Unmap all mmappings of the file up-front.
-	 *
-	 * This will cause any pte dirty bits to be propagated into the
-	 * pageframes for the subsequent filemap_write_and_wait().
-	 */
 	write_len = iov_length(iov, *nr_segs);
 	end = (pos + write_len - 1) >> PAGE_CACHE_SHIFT;
-	if (mapping_mapped(mapping))
-		unmap_mapping_range(mapping, pos, write_len, 0);
 
-	written = filemap_write_and_wait(mapping);
+	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
 	if (written)
 		goto out;
 
@@ -2286,7 +2279,8 @@ generic_file_buffered_write(struct kiocb
 	 * the file data here, to try to honour O_DIRECT expectations.
 	 */
 	if (unlikely(file->f_flags & O_DIRECT) && written)
-		status = filemap_write_and_wait(mapping);
+		status = filemap_write_and_wait_range(mapping,
+					pos, pos + written - 1);
 
 	return written ? written : status;
 }