Re: [PATCH] zram: fix incorrectly stat with failed_reads

[Minchan Kim]

On Mon, Aug 11, 2014 at 04:39:17PM +0800, Chao Yu wrote:
> Since we allocate a temporary buffer in zram_bvec_read to handle partial page
> operations in this commit 924bd88d703e53d30f393fac6117f8f1bc79aab6 (Staging:
> zram: allow partial page operations), our ->failed_reads value may be incorrect
> as we do not increase its value when failed to allocate the temporary buffer.
> 
> Let's fix this issue and correct the annotation of failed_reads.

Good catch.
Just out of curiosity.

Did you catch it by code review or real workload?

> 
> Signed-off-by: Chao Yu <chao2.yu@xxxxxxxxxxx>
> ---
>  drivers/block/zram/zram_drv.c | 1 +
>  drivers/block/zram/zram_drv.h | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index dfa4024..bf8ea1b 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -413,6 +413,7 @@ static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
>  
>  	if (!uncmem) {
>  		pr_info("Unable to allocate temp memory\n");
> +		atomic64_inc(&zram->stats.failed_reads);
>  		ret = -ENOMEM;
>  		goto out_cleanup;
>  	}
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 5b0afde..e0f725c 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -84,7 +84,7 @@ struct zram_stats {
>  	atomic64_t compr_data_size;	/* compressed size of pages stored */
>  	atomic64_t num_reads;	/* failed + successful */
>  	atomic64_t num_writes;	/* --do-- */
> -	atomic64_t failed_reads;	/* should NEVER! happen */
> +	atomic64_t failed_reads;	/* can happen when memory is too low */
>  	atomic64_t failed_writes;	/* can happen when memory is too low */
>  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
>  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> -- 
> 2.0.1.474.g72c7794

How abouting moving failed_reads/writes in zram_bvec_rw?

int zram_bvec_rw(..)
{
        if (rw == READ) {
                atomic64_inc(num_reads);
                ret = zram_bvec_read(xxx);
        } else {
                atomic64_inc(&num_writes);
                ret = zram_bvec_write(xxx);
        }

        if (unlikely(ret)) {
                if (rw == READ)
                        atomic64_inc(failed_reads);
                else
                        atomic64_inc(failed_writes);
        }
}

> 
> 
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-- 
Kind regards,
Minchan Kim
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