readahead: move the random read case to bottom

Split all readahead cases, and move the random one to bottom. No behavior changes. This is to prepare for the introduction of context readahead, and make it easy for inserting accounting/tracing points for each case. Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> Cc: Vladislav Bolkhovitin <vst@vlnb.net> Cc: Jens Axboe <jens.axboe@oracle.com> Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Cc: Ying Han <yinghan@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

readahead: move the random read case to bottom
Split all readahead cases, and move the random one to bottom. No behavior changes. This is to prepare for the introduction of context readahead, and make it easy for inserting accounting/tracing points for each case. Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> Cc: Vladislav Bolkhovitin <vst@vlnb.net> Cc: Jens Axboe <jens.axboe@oracle.com> Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Cc: Ying Han <yinghan@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Wu Fengguang · Linus Torvalds
1 parent dc566127dd
Showing 1 changed file with 25 additions and 21 deletions Side-by-side Diff
mm/readahead.c
@@ -339,34 +339,26 @@
 		   unsigned long req_size)
 {
 	unsigned long max = max_sane_readahead(ra->ra_pages);
-	pgoff_t prev_offset;
-	int	sequential;
  
 	/*
+	 * start of file
+	 */
+	if (!offset)
+		goto initial_readahead;
+
+	/*
 	 * It's the expected callback offset, assume sequential access.
 	 * Ramp up sizes, and push forward the readahead window.
 	 */
-	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
-			offset == (ra->start + ra->size))) {
+	if ((offset == (ra->start + ra->size - ra->async_size) ||
+	     offset == (ra->start + ra->size))) {
 		ra->start += ra->size;
 		ra->size = get_next_ra_size(ra, max);
 		ra->async_size = ra->size;
 		goto readit;
 	}
  
-	prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
-	sequential = offset - prev_offset <= 1UL || req_size > max;
-
 	/*
-	 * Standalone, small read.
-	 * Read as is, and do not pollute the readahead state.
-	 */
-	if (!hit_readahead_marker && !sequential) {
-		return __do_page_cache_readahead(mapping, filp,
-						offset, req_size, 0);
-	}
-
-	/*
 	 * Hit a marked page without valid readahead state.
 	 * E.g. interleaved reads.
 	 * Query the pagecache for async_size, which normally equals to
  
@@ -391,12 +383,24 @@
 	}
  
 	/*
-	 * It may be one of
-	 * 	- first read on start of file
-	 * 	- sequential cache miss
-	 * 	- oversize random read
-	 * Start readahead for it.
+	 * oversize read
 	 */
+	if (req_size > max)
+		goto initial_readahead;
+
+	/*
+	 * sequential cache miss
+	 */
+	if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
+		goto initial_readahead;
+
+	/*
+	 * standalone, small random read
+	 * Read as is, and do not pollute the readahead state.
+	 */
+	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+
+initial_readahead:
 	ra->start = offset;
 	ra->size = get_init_ra_size(req_size, max);
 	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
...	...	@@ -339,34 +339,26 @@
339	339	unsigned long req_size)
340	340	{
341	341	unsigned long max = max_sane_readahead(ra->ra_pages);
342		- pgoff_t prev_offset;
343		- int sequential;
344	342
345	343	/*
	344	+ * start of file
	345	+ */
	346	+ if (!offset)
	347	+ goto initial_readahead;
	348	+
	349	+ /*
346	350	* It's the expected callback offset, assume sequential access.
347	351	* Ramp up sizes, and push forward the readahead window.
348	352	*/
349		- if (offset && (offset == (ra->start + ra->size - ra->async_size) \|\|
350		- offset == (ra->start + ra->size))) {
	353	+ if ((offset == (ra->start + ra->size - ra->async_size) \|\|
	354	+ offset == (ra->start + ra->size))) {
351	355	ra->start += ra->size;
352	356	ra->size = get_next_ra_size(ra, max);
353	357	ra->async_size = ra->size;
354	358	goto readit;
355	359	}
356	360
357		- prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
358		- sequential = offset - prev_offset <= 1UL \|\| req_size > max;
359		-
360	361	/*
361		- * Standalone, small read.
362		- * Read as is, and do not pollute the readahead state.
363		- */
364		- if (!hit_readahead_marker && !sequential) {
365		- return __do_page_cache_readahead(mapping, filp,
366		- offset, req_size, 0);
367		- }
368		-
369		- /*
370	362	* Hit a marked page without valid readahead state.
371	363	* E.g. interleaved reads.
372	364	* Query the pagecache for async_size, which normally equals to
373	365
...	...	@@ -391,12 +383,24 @@
391	383	}
392	384
393	385	/*
394		- * It may be one of
395		- * - first read on start of file
396		- * - sequential cache miss
397		- * - oversize random read
398		- * Start readahead for it.
	386	+ * oversize read
399	387	*/
	388	+ if (req_size > max)
	389	+ goto initial_readahead;
	390	+
	391	+ /*
	392	+ * sequential cache miss
	393	+ */
	394	+ if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
	395	+ goto initial_readahead;
	396	+
	397	+ /*
	398	+ * standalone, small random read
	399	+ * Read as is, and do not pollute the readahead state.
	400	+ */
	401	+ return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
	402	+
	403	+initial_readahead:
400	404	ra->start = offset;
401	405	ra->size = get_init_ra_size(req_size, max);
402	406	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;