Re: [PATCH v2 3/3] CMA: always treat free cma pages as non-free on watermark checking

[Joonsoo Kim]

2014-05-30 19:40 GMT+09:00 Ritesh Harjani <ritesh.list@xxxxxxxxx>:
> Hi Joonsoo,
>
> I think you will be loosing the benefit of below patch with your changes.
> I am no expert here so please bear with me. I tried explaining in the
> inline comments, let me know if I am wrong.
>
> commit 026b08147923142e925a7d0aaa39038055ae0156
> Author: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
> Date:   Wed Jun 12 14:05:02 2013 -0700

Hello, Ritesh.

Thanks for notifying that.

>
> On Wed, May 28, 2014 at 12:34 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>> commit d95ea5d1('cma: fix watermark checking') introduces ALLOC_CMA flag
>> for alloc flag and treats free cma pages as free pages if this flag is
>> passed to watermark checking. Intention of that patch is that movable page
>> allocation can be be handled from cma reserved region without starting
>> kswapd. Now, previous patch changes the behaviour of allocator that
>> movable allocation uses the page on cma reserved region aggressively,
>> so this watermark hack isn't needed anymore. Therefore remove it.
>>
>> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 627dc2e..36e2fcd 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1117,10 +1117,6 @@ unsigned long try_to_compact_pages(struct zonelist *zonelist,
>>
>>         count_compact_event(COMPACTSTALL);
>>
>> -#ifdef CONFIG_CMA
>> -       if (allocflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
>> -               alloc_flags |= ALLOC_CMA;
>> -#endif
>>         /* Compact each zone in the list */
>>         for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx,
>>                                                                 nodemask) {
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 07b6736..a121762 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -384,7 +384,6 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
>>  #define ALLOC_HARDER           0x10 /* try to alloc harder */
>>  #define ALLOC_HIGH             0x20 /* __GFP_HIGH set */
>>  #define ALLOC_CPUSET           0x40 /* check for correct cpuset */
>> -#define ALLOC_CMA              0x80 /* allow allocations from CMA areas */
>> -#define ALLOC_FAIR             0x100 /* fair zone allocation */
>> +#define ALLOC_FAIR             0x80 /* fair zone allocation */
>>
>>  #endif /* __MM_INTERNAL_H */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index ca678b6..83a8021 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1764,20 +1764,22 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>>         long min = mark;
>>         long lowmem_reserve = z->lowmem_reserve[classzone_idx];
>>         int o;
>> -       long free_cma = 0;
>>
>>         free_pages -= (1 << order) - 1;
>>         if (alloc_flags & ALLOC_HIGH)
>>                 min -= min / 2;
>>         if (alloc_flags & ALLOC_HARDER)
>>                 min -= min / 4;
>> -#ifdef CONFIG_CMA
>> -       /* If allocation can't use CMA areas don't use free CMA pages */
>> -       if (!(alloc_flags & ALLOC_CMA))
>> -               free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>> -#endif
>> +       /*
>> +        * We don't want to regard the pages on CMA region as free
>> +        * on watermark checking, since they cannot be used for
>> +        * unmovable/reclaimable allocation and they can suddenly
>> +        * vanish through CMA allocation
>> +        */
>> +       if (IS_ENABLED(CONFIG_CMA) && z->managed_cma_pages)
>> +               free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
>
> make this free_cma instead of free_pages.
>
>>
>> -       if (free_pages - free_cma <= min + lowmem_reserve)
>> +       if (free_pages <= min + lowmem_reserve)
> free_pages - free_cma <= min + lowmem_reserve
>
> Because in for loop you subtract nr_free which includes the CMA pages.
> So if you have subtracted NR_FREE_CMA_PAGES
> from free_pages above then you will be subtracting cma pages again in
> nr_free (below in for loop).

Yes, I understand the problem you mentioned.

I think that this is complicated issue.

Comit '026b081' you mentioned makes watermark_ok() loose for high order
allocation compared to kernel that CMA isn't enabled, since free_pages includes
free_cma pages and most of high order allocation except THP would be
non-movable allocation. This non-movable allocation can't use cma pages,
so we shouldn't include free_cma pages.

If most of free cma pages are 0 order, that commit works correctly. We subtract
nr of free cma pages at the first loop, so there is no problem. But,
if the system
have some free high-order cma pages, watermark checking allow high-order
allocation more easily.

I think that loosing the watermark check is right solution so will takes your
comment on v2. But I want to know other developer's opinion.
If needed, I can implement to track free_area[o].nr_cma_free and use it for
precise freepage calculation in watermark check.

Thanks.
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