readahead: combine file_ra_state.prev_index/prev_offset into prev_pos

Combine the file_ra_state members
				unsigned long prev_index
				unsigned int prev_offset
into
				loff_t prev_pos

It is more consistent and better supports huge files.

Thanks to Peter for the nice proposal!

[akpm@linux-foundation.org: fix shift overflow]
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 15 insertions, 13 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 5dc18d76e703..bbcca456d8a6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -879,8 +879,8 @@ void do_generic_mapping_read(struct address_space *mapping,
         cached_page = NULL;
         index = *ppos >> PAGE_CACHE_SHIFT;
         next_index = index;
-        prev_index = ra.prev_index;
-        prev_offset = ra.prev_offset;
+        prev_index = ra.prev_pos >> PAGE_CACHE_SHIFT;
+        prev_offset = ra.prev_pos & (PAGE_CACHE_SIZE-1);
         last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
         offset = *ppos & ~PAGE_CACHE_MASK;
 
@@ -966,7 +966,6 @@ page_ok:
                 index += offset >> PAGE_CACHE_SHIFT;
                 offset &= ~PAGE_CACHE_MASK;
                 prev_offset = offset;
-                ra.prev_offset = offset;
 
                 page_cache_release(page);
                 if (ret == nr && desc->count)
@@ -1056,9 +1055,11 @@ no_cached_page:
 
 out:
         *_ra = ra;
-        _ra->prev_index = prev_index;
+        _ra->prev_pos = prev_index;
+        _ra->prev_pos <<= PAGE_CACHE_SHIFT;
+        _ra->prev_pos |= prev_offset;
 
-        *ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
+        *ppos = ((loff_t)index << PAGE_CACHE_SHIFT) + offset;
         if (cached_page)
                 page_cache_release(cached_page);
         if (filp)
@@ -1396,7 +1397,7 @@ retry_find:
          * Found the page and have a reference on it.
          */
         mark_page_accessed(page);
-        ra->prev_index = page->index;
+        ra->prev_pos = (loff_t)page->index << PAGE_CACHE_SHIFT;
         vmf->page = page;
         return ret | VM_FAULT_LOCKED;
 
diff --git a/mm/readahead.c b/mm/readahead.c
index d2504877b269..4a58befbde4a 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -46,7 +46,7 @@ void
 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 {
         ra->ra_pages = mapping->backing_dev_info->ra_pages;
-        ra->prev_index = -1;
+        ra->prev_pos = -1;
 }
 EXPORT_SYMBOL_GPL(file_ra_state_init);
 
@@ -327,7 +327,7 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
  * indicator. The flag won't be set on already cached pages, to avoid the
  * readahead-for-nothing fuss, saving pointless page cache lookups.
  *
- * prev_index tracks the last visited page in the _previous_ read request.
+ * prev_pos tracks the last visited byte in the _previous_ read request.
  * It should be maintained by the caller, and will be used for detecting
  * small random reads. Note that the readahead algorithm checks loosely
  * for sequential patterns. Hence interleaved reads might be served as
@@ -351,11 +351,9 @@ ondemand_readahead(struct address_space *mapping,
                    bool hit_readahead_marker, pgoff_t offset,
                    unsigned long req_size)
 {
-        int max;        /* max readahead pages */
-        int sequential;
-
-        max = ra->ra_pages;
-        sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
+        int     max = ra->ra_pages;     /* max readahead pages */
+        pgoff_t prev_offset;
+        int     sequential;
 
         /*
          * It's the expected callback offset, assume sequential access.
@@ -369,6 +367,9 @@ ondemand_readahead(struct address_space *mapping,
                 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.