readahead: improve heuristic detecting sequential reads

Introduce ra.offset and store in it an offset where the previous read
ended.  This way we can detect whether reads are really sequential (and
thus we should not mark the page as accessed repeatedly) or whether they
are random and just happen to be in the same page (and the page should
really be marked accessed again).

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: WU Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 9 insertions, 3 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index cbea95a25283..07f5b77114a3 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -868,6 +868,7 @@ void do_generic_mapping_read(struct address_space *mapping,
         unsigned long last_index;
         unsigned long next_index;
         unsigned long prev_index;
+        unsigned int prev_offset;
         loff_t isize;
         struct page *cached_page;
         int error;
@@ -877,6 +878,7 @@ void do_generic_mapping_read(struct address_space *mapping,
         index = *ppos >> PAGE_CACHE_SHIFT;
         next_index = index;
         prev_index = ra.prev_page;
+        prev_offset = ra.offset;
         last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
         offset = *ppos & ~PAGE_CACHE_MASK;
 
@@ -924,10 +926,10 @@ page_ok:
                         flush_dcache_page(page);
 
                 /*
-                 * When (part of) the same page is read multiple times
-                 * in succession, only mark it as accessed the first time.
+                 * When a sequential read accesses a page several times,
+                 * only mark it as accessed the first time.
                  */
-                if (prev_index != index)
+                if (prev_index != index || offset != prev_offset)
                         mark_page_accessed(page);
                 prev_index = index;
 
@@ -945,6 +947,7 @@ page_ok:
                 offset += ret;
                 index += offset >> PAGE_CACHE_SHIFT;
                 offset &= ~PAGE_CACHE_MASK;
+                prev_offset = ra.offset = offset;
 
                 page_cache_release(page);
                 if (ret == nr && desc->count)
diff --git a/mm/readahead.c b/mm/readahead.c
index 93d9ee692fd8..0a6fed9d365c 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -207,6 +207,8 @@ out:
  *              If page_cache_readahead sees that it is again being called for
  *              a page which it just looked at, it can return immediately without
  *              making any state changes.
+ * offset:      Offset in the prev_page where the last read ended - used for
+ *              detection of sequential file reading.
  * ahead_start,
  * ahead_size:  Together, these form the "ahead window".
  * ra_pages:    The externally controlled max readahead for this fd.
@@ -473,6 +475,7 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
         /* Note that prev_page == -1 if it is a first read */
         sequential = (offset == ra->prev_page + 1);
         ra->prev_page = offset;
+        ra->offset = 0;
 
         max = get_max_readahead(ra);
         newsize = min(req_size, max);