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SubjectRe: [PATCH 0/5] zswap: make params runtime changeable
On Tue, Jun 2, 2015 at 4:26 PM, Seth Jennings <sjennings@variantweb.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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