Re: [PATCH 3/3] [RFC] tmpfs: Add FALLOC_FL_MARK_VOLATILE/UNMARK_VOLATILEhandlers

[Minchan Kim]

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?

> 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)

Off-topic:

I want to drive low memory notification level-triggering instead of raw vmstat trigger.
(It's rather long thread https://lkml.org/lkml/2012/5/1/97)

level 1: out-of-easy reclaimable pages (NR_VOLATILE + NR_UNMAPPED_CLEAN_PAGE)
level 2 (more sever VM pressure than level 1): level2 + reclaimable dirty pages

When it is out of easy-reclaimable pages, it might be good indication for
low memory notification.


> 
> 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.

Off-topic: 
But I'm not sure because I might try to make new easy-reclaimable LRU list for low memory notification.
That LRU list would contain non-mapped clean cache page and volatile pages if I decide adding it.
Both pages has a common characteristic that recreating page is less costly.
It's true for eMMC/SSD like device, at least.

> 
> Does that sound reasonable?  Any other suggested approaches?  I'll think
> some more about it this weekend and try to get a patch scratched out
> early next week.
> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim
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