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SubjectRe: [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.


> 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.


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