Auto-detect sequential mmap reads and do sync/async readahead for them.

The sequential mmap readahead will be triggered when
- sync readahead: it's a major fault and (prev_offset==offset-1);
- async readahead: minor fault on PG_readahead page with valid readahead state.

It's a bit conservative to require valid readahead state for async readahead,
which means we don't do readahead for interleaved reads for now, but let's make
it safe for this initial try.

Signed-off-by: Fengguang Wu
---

---
mm/filemap.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-2.6.24-rc4-mm1.orig/mm/filemap.c
+++ linux-2.6.24-rc4-mm1/mm/filemap.c
@@ -1318,7 +1318,8 @@ static void do_sync_mmap_readahead(struc
if (VM_RandomReadHint(vma))
return;

- if (VM_SequentialReadHint(vma)) {
+ if (VM_SequentialReadHint(vma) ||
+ offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
page_cache_sync_readahead(mapping, ra, file, offset, 1);
return;
}
@@ -1360,7 +1361,8 @@ static void do_async_mmap_readahead(stru
return;
if (ra->mmap_miss > 0)
ra->mmap_miss--;
- if (PageReadahead(page))
+ if (PageReadahead(page) &&
+ offset == ra->start + ra->size - ra->async_size)
page_cache_async_readahead(mapping, ra, file, page, offset, 1);
}


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