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Date: Fri, 3 Aug 2018 11:52:30 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tino Lehnig <tino.lehnig@contabo.de>, stable@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 1/2] zram: remove BD_CAP_SYNCHRONOUS_IO with writeback
 feature
Message-ID: <20180803025230.GA502@jagdpanzerIV>
References: <20180802051112.86174-1-minchan@kernel.org>
 <20180802141304.d0589ddc5f8213429ab3b565@linux-foundation.org>
 <20180803023929.GA7500@jagdpanzerIV>
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On (08/03/18 11:39), Sergey Senozhatsky wrote:
> [..]
>
> > A reader looking at this would wonder "why the heck are we doing that".
> > Adding a code comment would help them.
> 
> The interesting thing here is that include/linux/backing-dev.h
> BDI_CAP_SYNCHRONOUS_IO comment says
> 
> 	"Device is so fast that asynchronous IO would be inefficient."
> 
> Which is not the reason why BDI_CAP_SYNCHRONOUS_IO is used by ZRAM.
> Probably, the comment needs to be updated as well.
> 
> Both SWP_SYNCHRONOUS_IO and BDI_CAP_SYNCHRONOUS_IO tend to pivot
> "efficiency" [looking at the comments], but in ZRAM's case the whole
> reason to use SYNC IO is a race condition and user-after-free that
               ^ASYNC IO

	-ss