Date:	Tue, 2 Jun 2015 16:47:03 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	Seth Jennings <sjennings@...iantweb.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] zswap: make params runtime changeable

On Tue, Jun 2, 2015 at 4:26 PM, Seth Jennings <sjennings@...iantweb.net> wrote:
> On Tue, Jun 02, 2015 at 11:11:52AM -0400, Dan Streetman wrote:
>> This patch series allows setting all zswap params at runtime, instead
>> of only being settable at boot-time.
>>
>> The changes to zswap are rather large, due to the creation of zswap pools,
>> which contain both a compressor function as well as a zpool.  When either
>> the compressor or zpool param is changed at runtime, a new zswap pool is
>> created with the new compressor and zpool, and used for all new compressed
>> pages.  Any old zswap pools that still contain pages are retained only to
>> load pages from, and destroyed once they become empty.
>>
>> One notable change required for this to work is to split the currently
>> global kernel param mutex into a global mutex only for built-in params,
>> and a per-module mutex for loadable module params.  The reason this change
>> is required is because zswap's compressor and zpool param handler callback
>> functions attempt to load, via crypto_has_comp() and the new zpool_has_pool()
>> functions, any required compressor or zpool modules.  The problem there is
>> that the zswap param callback functions run while the global param mutex is
>> locked, but when they attempt to load another module, if the loading module
>> has any params set e.g. via /etc/modprobe.d/*.conf, modprobe will also try
>> to take the global param mutex, and a deadlock will result, with the mutex
>> held by the zswap param callback which is waiting for modprobe, but modprobe
>> waiting for the mutex to change the loading module's param.  Using a
>> per-module mutex for all loadable modules prevents this, since each module
>> will take its own mutex and never conflict with another module's param
>> changes.
>
> Nice work Dan :)
>
> I'm trying to look at this as three different efforts. In order of
> increasing difficulty:
> - Enabling/disabling zswap at runtime
> - Changing the compressor at runtime, which doesn't involve the zpool layer
> - Changing the allocator (type) at runtime which does involve the zpool layer.
>
> In other words, we can store entries that use a different compressor in
> the same zpool, but not entries stored in different allocators.
>
> Enabling zswap at runtime is very straightforward, especially if you
> aren't going to attempt to flush out all the pages on a disable; only
> prevent new stores.  I like that.
>
> Changing the compressor at runtime is the next easiest one, since you
> have to allocate new compressor transforms, but not a new zpool.  You
> just store which compressor was used on a per-entry basis.
>
> Changing the allocator (type) is the hardest since it involves a new
> zpool, and all the code for managing multiple zpools in zswap.
>
> This is a lot of change all at once.  Maybe we could just do the runtime
> enable/disable of zswap and the runtime change of compressors first?  I
> think those two alone would be a lot less invasive.  Then we can look at
> runtime change of the allocator as a separate thing.

Sure I'll send the enable/disable individually first, with doc updates.

I'll send the other patches as well, to consider separately.

>
> Thanks,
> Seth
>
>>
>>
>> Dan Streetman (5):
>>   zpool: add zpool_has_pool()
>>   module: add per-module params lock
>>   zswap: runtime enable/disable
>>   zswap: dynamic pool creation
>>   zswap: change zpool/compressor at runtime
>>
>>  arch/um/drivers/hostaudio_kern.c                 |  20 +-
>>  drivers/net/ethernet/myricom/myri10ge/myri10ge.c |   6 +-
>>  drivers/net/wireless/libertas_tf/if_usb.c        |   6 +-
>>  drivers/usb/atm/ueagle-atm.c                     |   4 +-
>>  drivers/video/fbdev/vt8623fb.c                   |   4 +-
>>  include/linux/module.h                           |   1 +
>>  include/linux/moduleparam.h                      |  67 +--
>>  include/linux/zpool.h                            |   2 +
>>  kernel/module.c                                  |   1 +
>>  kernel/params.c                                  |  45 +-
>>  mm/zpool.c                                       |  25 +
>>  mm/zswap.c                                       | 696 +++++++++++++++++------
>>  net/mac80211/rate.c                              |   4 +-
>>  13 files changed, 640 insertions(+), 241 deletions(-)
>>
>> --
>> 2.1.0
>>
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