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    Subject[PATCH 3/3 with new changelog] readahead: trigger mmap sequential readahead on PG_readahead
    Previously the mmap sequential readahead is triggered by updating
    ra->prev_pos on each page fault and compare it with current page offset.

    In the mosbench exim benchmark which does multi-threaded page faults on
    shared struct file, the ra->mmap_miss and ra->prev_pos updates are found
    to cause excessive cache line bouncing on tmpfs, which actually disabled
    readahead totally (shmem_backing_dev_info.ra_pages == 0).

    So remove the ra->prev_pos recording, and instead tag PG_readahead to
    trigger the possible sequential readahead. It's not only more simple,
    but also will work more reliably on concurrent reads on shared struct file.

    Tested-by: Tim Chen <tim.c.chen@intel.com>
    Reported-by: Andi Kleen <ak@linux.intel.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/filemap.c | 6 ++----
    1 file changed, 2 insertions(+), 4 deletions(-)

    --- linux-next.orig/mm/filemap.c 2011-04-23 16:52:21.000000000 +0800
    +++ linux-next/mm/filemap.c 2011-04-24 09:59:08.000000000 +0800
    @@ -1531,8 +1531,7 @@ static void do_sync_mmap_readahead(struc
    if (!ra->ra_pages)
    return;

    - if (VM_SequentialReadHint(vma) ||
    - offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
    + if (VM_SequentialReadHint(vma)) {
    page_cache_sync_readahead(mapping, ra, file, offset,
    ra->ra_pages);
    return;
    @@ -1555,7 +1554,7 @@ static void do_sync_mmap_readahead(struc
    ra_pages = max_sane_readahead(ra->ra_pages);
    ra->start = max_t(long, 0, offset - ra_pages / 2);
    ra->size = ra_pages;
    - ra->async_size = 0;
    + ra->async_size = ra_pages / 4;
    ra_submit(ra, mapping, file);
    }

    @@ -1661,7 +1660,6 @@ retry_find:
    return VM_FAULT_SIGBUS;
    }

    - ra->prev_pos = (loff_t)offset << PAGE_CACHE_SHIFT;
    vmf->page = page;
    return ret | VM_FAULT_LOCKED;


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