Re: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILE handlers

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(6/7/12 11:03 PM), John Stultz wrote:
> On 06/07/2012 04:41 PM, Dave Hansen wrote:
>> On 06/07/2012 03:55 AM, Dmitry Adamushko wrote:
>>> but maybe we should also purge them before we swap out some non-tmpfs
>>> pages or drop some file-backed pages?
>>
>> Sure... I guess we could kick that from either direct reclaim or from
>> kswapd. But, then we're basically back to the places where
>> shrink_slab() is called.
>>
>> I think that means that we think it's preferable to integrate this more
>> directly in the VM instead of sticking it off in the corner of tmpfs
>> only, or pretending it's a slab.
>>
>> Dunno... The slab shrinker one isn't looking _so_ bad at the moment.
>
> Dave also pointed out to me on irc that on a system without swap,
>shmem_writepage doesn't even get called, which kills the utility of
>triggering volatile purging from writepage.

Ah, right you are. swap-less system never try to reclaim anon pages. So,
volatile pages is no longer swap backed. swap backed lru is no longer suitable
place.

  
> So I'm falling back to using a shrinker for now, but I think Dmitry's
>point is an interesting one, and am interested in finding a better
>place to trigger purging volatile ranges from the mm code. If anyone has any
>suggestions, let me know, otherwise I'll go back to trying to better grok the mm code.

I hate vm feature to abuse shrink_slab(). because of, it was not designed generic callback.
it was designed for shrinking filesystem metadata. Therefore, vm keeping a balance between
page scanning and slab scanning. then, a lot of shrink_slab misuse may lead to break balancing
logic. i.e. drop icache/dcache too many and makes perfomance impact.

As far as a code impact is small, I'm prefer to connect w/ vm reclaim code directly.