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

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Message-ID: <20180803023929.GA7500@jagdpanzerIV>
Date:   Fri, 3 Aug 2018 11:39:29 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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

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.

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.

	-ss