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    SubjectRe: [PATCH 3/8] readahead: replace ra->mmap_miss with ra->ra_flags
    On Mon, Nov 21, 2011 at 03:01:16PM -0800, Andrew Morton wrote:
    > On Mon, 21 Nov 2011 17:18:22 +0800
    > Wu Fengguang <fengguang.wu@intel.com> wrote:
    >
    > > Introduce a readahead flags field and embed the existing mmap_miss in it
    > > (mainly to save space).
    >
    > What an ugly patch.

    Indeed..

    > > It will be possible to lose the flags in race conditions, however the
    > > impact should be limited. For the race to happen, there must be two
    > > threads sharing the same file descriptor to be in page fault or
    > > readahead at the same time.
    > >
    > > Note that it has always been racy for "page faults" at the same time.
    > >
    > > And if ever the race happen, we'll lose one mmap_miss++ or mmap_miss--.
    > > Which may change some concrete readahead behavior, but won't really
    > > impact overall I/O performance.
    > >
    > > CC: Andi Kleen <andi@firstfloor.org>
    > > CC: Steven Whitehouse <swhiteho@redhat.com>
    > > Acked-by: Rik van Riel <riel@redhat.com>
    > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    > > ---
    > > include/linux/fs.h | 31 ++++++++++++++++++++++++++++++-
    > > mm/filemap.c | 9 ++-------
    > > 2 files changed, 32 insertions(+), 8 deletions(-)
    > >
    > > --- linux-next.orig/include/linux/fs.h 2011-11-20 11:30:55.000000000 +0800
    > > +++ linux-next/include/linux/fs.h 2011-11-20 11:48:53.000000000 +0800
    > > @@ -945,10 +945,39 @@ struct file_ra_state {
    > > there are only # of pages ahead */
    > >
    > > unsigned int ra_pages; /* Maximum readahead window */
    > > - unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
    > > + unsigned int ra_flags;
    >
    > And it doesn't actually save any space, unless ra_flags gets used for
    > something else in a subsequent patch. And if it does, perhaps ra_flags

    Because it's a preparation patch. There will be more fields defined later.

    > should be ulong, which is compatible with the bitops.h code.
    > Or perhaps we should use a bitfield and let the compiler do the work.

    What if we do

    u16 mmap_miss;
    u16 ra_flags;

    That would get rid of this patch. I'd still like to pack the various
    flags as well as pattern into one single ra_flags, which makes it
    convenient to pass things around (as one single parameter).

    > > loff_t prev_pos; /* Cache last read() position */
    > > };
    > >
    > > +/* ra_flags bits */
    > > +#define READAHEAD_MMAP_MISS 0x000003ff /* cache misses for mmap access */
    > > +
    > > +/*
    > > + * Don't do ra_flags++ directly to avoid possible overflow:
    > > + * the ra fields can be accessed concurrently in a racy way.
    > > + */
    > > +static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra)
    > > +{
    > > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
    > > +
    > > + /* the upper bound avoids banging the cache line unnecessarily */
    > > + if (miss < READAHEAD_MMAP_MISS) {
    > > + miss++;
    > > + ra->ra_flags = miss | (ra->ra_flags & ~READAHEAD_MMAP_MISS);
    > > + }
    > > + return miss;
    > > +}
    > > +
    > > +static inline void ra_mmap_miss_dec(struct file_ra_state *ra)
    > > +{
    > > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
    > > +
    > > + if (miss) {
    > > + miss--;
    > > + ra->ra_flags = miss | (ra->ra_flags & ~READAHEAD_MMAP_MISS);
    > > + }
    > > +}
    >
    > It's strange that ra_mmap_miss_inc() returns the new value whereas
    > ra_mmap_miss_dec() returns void.

    Simply because no one need to check the return value of ra_mmap_miss_dec()...
    But yeah it's good to make them look symmetry.

    Thanks,
    Fengguang


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