linux-kernel - Re: [PATCH 2/2] zram: do not count duplicated pages as compressed

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Date:   Sun, 21 May 2017 00:04:27 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        kernel-team <kernel-team@....com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, hch@...radead.org,
        viro@...iv.linux.org.uk, axboe@...nel.dk, jlayton@...hat.com,
        tytso@....edu
Subject: Re: [PATCH 2/2] zram: do not count duplicated pages as compressed

On Wed, May 17, 2017 at 05:32:12PM +0900, Minchan Kim wrote:
> Is block device(esp, zram which is compressed ram block device) okay to
> return garbage when ongoing overwrite IO fails?
> 
> O_DIRECT write 4 block "aaa.." -> success
> read  4 block "aaa.." -> success
> O_DIRECT write 4 block "bbb.." -> fail
> read  4 block "000..' -> it is okay?
> 
> Hope to get an answer form experts. :)

It's "okay" as it's what existing real block devices do (at least on a
sector boundary).  It's not "nice" though, so if you can avoid it,
please do.