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    SubjectRe: [PATCH][1/2]page_fault retry with NOPAGE_RETRY
    On Fri, Apr 10, 2009 at 02:32:01PM +0800, Ying Han wrote:
    > 2009/4/9 Andrew Morton <akpm@linux-foundation.org>:
    > >
    > >> Subject: [PATCH][1/2]page_fault retry with NOPAGE_RETRY
    > >
    > > Please give each patch in the series a unique and meaningful title.
    > >
    > > On Wed, 8 Apr 2009 13:02:35 -0700 Ying Han <yinghan@google.com> wrote:
    > >
    > >> support for FAULT_FLAG_RETRY with no user change:
    > >
    > > yup, we'd prefer a complete changelog here please.
    > >
    > >> Signed-off-by: Ying Han <yinghan@google.com>
    > >> Mike Waychison <mikew@google.com>
    > >
    > > This form:
    > >
    > > Signed-off-by: Ying Han <yinghan@google.com>
    > > Signed-off-by: Mike Waychison <mikew@google.com>
    >
    > Thanks Andrew, and i need to add Fengguang to Signed-off-by.

    Thank you.

    > >
    > > is conventional.
    > >
    > >> index 4a853ef..29c2c39 100644
    > >> --- a/include/linux/fs.h
    > >> +++ b/include/linux/fs.h
    > >> @@ -793,7 +793,7 @@ struct file_ra_state {
    > >> there are only # of pages ahead */
    > >>
    > >> unsigned int ra_pages; /* Maximum readahead window */
    > >> - int mmap_miss; /* Cache miss stat for mmap accesses */
    > >> + unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
    > >
    > > This change makes sense, but we're not told the reasons for making it?
    > > Did it fix a bug, or is it an unrelated fixlet, or...?
    >
    > Fengguang: Could you help making comments on this part? and i will
    > make changes elsewhere as Andrew pointed. Thanks

    Ah this may deserve a standalone patch:
    ---
    readhead: make mmap_miss an unsigned int

    This makes the performance impact of possible mmap_miss wrap around to be
    temporary and tolerable: i.e. MMAP_LOTSAMISS=100 extra readarounds.

    Otherwise if ever mmap_miss wraps around to negative, it takes INT_MAX
    cache misses to bring it back to normal state. During the time mmap
    readaround will be _enabled_ for whatever wild random workload. That's
    almost permanent performance impact.

    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    include/linux/fs.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- mm.orig/include/linux/fs.h
    +++ mm/include/linux/fs.h
    @@ -824,7 +824,7 @@ struct file_ra_state {
    there are only # of pages ahead */

    unsigned int ra_pages; /* Maximum readahead window */
    - int mmap_miss; /* Cache miss stat for mmap accesses */
    + unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
    loff_t prev_pos; /* Cache last read() position */
    };


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