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

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Date:   Fri, 3 Aug 2018 14:23:06 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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

On (08/03/18 13:51), Minchan Kim wrote:
> 
> AFAIK, onging writeback page couldn't freed so it was not writeabck problem.
> 
> What I'm tryig to fix is read part.
> If we use swapcache, it shouldn't be a problem either because swapcache
> has a reference count and we should wait PG_lock release before the freeing
> from the swapcache so there is no race condition.

Hmm, any chance a WB device can be async on its own? We add a page
to a new bio and submit it to another async device driver. Then we
return back to the upper layer (swap), which can free a page before
the device picks up a request. Can this happen?

[..]
> However, I decided, at this moment, going this simple way for
> stable-material to solve #0 and #1 problems at the same time.

Agreed. Thanks.

	-ss