linux-kernel - Re: [PATCH] zram: clear disk io accounting when reset zram device

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Message-ID: <20150530041638.GA525@swordfish>
Date:	Sat, 30 May 2015 13:16:38 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Weijie Yang <weijie.yang@...sung.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>, ngupta@...are.org,
	'Weijie Yang' <weijie.yang.kh@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] zram: clear disk io accounting when reset zram device

On (05/29/15 23:54), Minchan Kim wrote:
> I think the problem is caused from weired feature "reset" of zram.

agree.

> Until a while ago, we didn't have hot_add/del feature so we should
> use custom reset function but now we have hot/add feature.
> So reset is logically same feature(ie, reset = hot_remove+hot_add
> but remains same device id).
> 

hm, sounds interesting, but I think it will end up being tricky.

zram_remove() will be called from device's sysfs node (now we call it from
zram_control sysfs class node, makes a huge difference). sysfs locks the node
until node's read/write handler returns back, so zram_remove() will be called
with lock(s_active#XXX) being locked (we had a lockdep splat with these locks
recently), while zram_remove()->sysfs_remove_group() will once again attempt
to lock this node (the very same lock(s_active#XXX)). in other words, we cannot
fully remove zram device from its sysfs attr. and I don't want to add any bool
flags to zram_remove() and zram_add() indicating that this is a "partial" device
remove: don't delete device's sysfs group in remove() and don't create it in add().

doing reset from zram_control is easy, for sure:
	lock idr mutex,
	do zram_remove() and zram_add()
	unlock idr lock.

`echo ID > /sys/.../zram_control/reset`

no need to modify remove()/add() -- idr will pick up just released idx,
so device_id will be preserved. but it'll be hard to drop the per-device
`reset` attr and to make it a zram_control attr. things would have been
much simpler if all of zram users were also zramctl users. zramctl, from
this point of view, lets us change zram interfaces easily -- we merely need
to teach/modify zramctl, the rest is transparent.

	-ss
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