linux-kernel - Re: [PATCH 1/2] zram: remove BD_CAP_SYNCHRONOUS

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Date:   Fri, 3 Aug 2018 12:00:19 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tino Lehnig <tino.lehnig@...tabo.de>, stable@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 1/2] zram: remove BD_CAP_SYNCHRONOUS_IO with writeback
 feature

Hi Sergey,

On Fri, Aug 03, 2018 at 11:39:29AM +0900, Sergey Senozhatsky wrote:
> On (08/02/18 14:13), Andrew Morton wrote:
> [..]
> > That changelog is rather hard to follow.  Please review my edits:
> > 
> > : If zram supports writeback feature, it's no longer a BD_CAP_SYNCHRONOUS_IO
> 							^BDI_CAP_SYNCHRONOUS_IO
> 
> [..]
> 
> > 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.

I couldn't catch your point. Could you clarify a little bit more?
What do you want to correct for the comment?

> 
> 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
> follows.

Actually, it's not whole reason. As I wrote down, without it, swap_readpage
waits the IO completion for a long time by blk_poll so it causes system
sluggish problem when device is slow(e.g., zram with backing device).

Thanks.