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

[John Stultz <john.stultz@linaro.org>]

On 06/12/2012 12:16 AM, Minchan Kim wrote:
> Please, Cced linux-mm.
>
> On 06/09/2012 12:45 PM, John Stultz wrote:
>
>> On 06/07/2012 09:50 PM, KOSAKI Motohiro wrote:
>>> (6/7/12 11:03 PM), John Stultz wrote:
>>>
>>>> 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.
>> I can see your concern about mis-using the shrinker code. Also your
>> other email's point about the problem of having LRU range purging
>> behavior on a NUMA system makes some sense too.  Unfortunately I'm not
>> yet familiar enough with the reclaim core to sort out how to best track
>> and connect the volatile range purging in the vm's reclaim core yet.
>>
>> So for now, I've moved the code back to using the shrinker (along with
>> fixing a few bugs along the way).
>> Thus, currently we manage the ranges as so:
>>      [per fs volatile range lru head] ->  [volatile range] ->  [volatile
>> range] ->  [volatile range]
>> With the per-fs shrinker zaping the volatile ranges from the lru.
>>
>> I *think* ideally, the pages in a volatile range should be similar to
>> non-dirty file-backed pages.  There is a cost to restore them, but
>> freeing them is very cheap.  The trick is that volatile ranges
>> introduces a new relationship between pages. Since the neighboring
>> virtual pages in a volatile range are in effect tied together, purging
>> one effectively ruins the value of keeping the others, regardless of
>> which zone they are physically.
>>
>> So maybe the right appraoch give up the per-fs volatile range lru, and
>> try a varient of what DaveC and DaveH have suggested: Letting the page
>> based lru reclamation handle the selection on a physical page basis, but
>> then zapping the entirety of the neighboring range if any one page is
>> reclaimed.  In order to try to preserve the range based LRU behavior,
>> activate all the pages in the range together when the range is marked
>
> You mean deactivation for fast reclaiming, not activation when memory pressure happen?
Yes. Sorry for mixing up terms here. The point is moving all the pages together to the inactive list to preserve relative LRU behavior for purging ranges.



>> volatile.  Since we assume ranges are un-touched when volatile, that
>> should preserve LRU purging behavior on single node systems and on
>> multi-node systems it will approximate fairly closely.
>>
>> My main concern with this approach is marking and unmarking volatile
>> ranges needs to be fast, so I'm worried about the additional overhead of
>> activating each of the containing pages on mark_volatile.
>
> Yes. it could be a problem if range is very large and populated already.
> Why can't we make new hooks?
>
> Just concept for showing my intention..
>
> +int shrink_volatile_pages(struct zone *zone)
> +{
> +       int ret = 0;
> +       if (zone_page_state(zone, NR_ZONE_VOLATILE))
> +               ret = shmem_purge_one_volatile_range();
> +       return ret;
> +}
> +
>   static void shrink_zone(struct zone *zone, struct scan_control *sc)
>   {
>          struct mem_cgroup *root = sc->target_mem_cgroup;
> @@ -1827,6 +1835,18 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
>                  .priority = sc->priority,
>          };
>          struct mem_cgroup *memcg;
> +       int ret;
> +
> +       /*
> +        * Before we dive into trouble maker, let's look at easy-
> +        * reclaimable pages and avoid costly-reclaim if possible.
> +        */
> +       do {
> +               ret = shrink_volatile_pages();
> +               if (ret)
> +                       zone_watermark_ok(zone, sc->order, xxx);
> +                               return;
> +       } while(ret)

Hmm. I'm confused.
This doesn't seem that different from the shrinker approach.
How does this resolve the numa-unawareness issue that Kosaki-san brought up?


>> The other question I have with this approach is if we're on a system
>> that doesn't have swap, it *seems* (not totally sure I understand it
>> yet) the tmpfs file pages will be skipped over when we call
>> shrink_lruvec.  So it seems we may need to add a new lru_list enum and
>> nr[] entry (maybe LRU_VOLATILE?).   So then it may be that when we mark
>> a range as volatile, instead of just activating it, we move it to the
>> volatile lru, and then when we shrink from that list, we call back to
>> the filesystem to trigger the entire range purging.
> Adding new LRU idea might make very slow fallocate(VOLATILE) so I hope we can avoid that if possible.

Indeed. This is a major concern. I'm currently prototyping it out so I 
have a concrete sense of the performance cost.

thanks
-john

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