On Mon, 21 Nov 2011 17:18:22 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> Introduce a readahead flags field and embed the existing mmap_miss in it
> (mainly to save space).
What an ugly patch.
> 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@xxxxxxxxxxxxxx>
> CC: Steven Whitehouse <swhiteho@xxxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> 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
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.
> 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.
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
