Re: [PATCH 3/8] readahead: replace ra->mmap_miss with ra->ra_flags

[Andrew Morton <akpm@linux-foundation.org>]

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.

> 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
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.


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