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

[Joonsoo Kim <js1304@gmail.com>]

2014-06-02 19:47 GMT+09:00 Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>:
>
> Hi,
>
> On Monday, June 02, 2014 09:37:49 AM Ritesh Harjani wrote:
>> Hi Joonsoo,
>>
>> CC'ing the developer of the patch (Tomasz Stanislawski)
>>
>>
>> On Fri, May 30, 2014 at 8:16 PM, Joonsoo Kim <js1304@gmail.com> wrote:
>> > 2014-05-30 19:40 GMT+09:00 Ritesh Harjani <ritesh.list@gmail.com>:
>> >> 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@samsung.com>
>> >> 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@lge.com> wrote:
>> >>> commit d95ea5d1('cma: fix watermark checking') introduces ALLOC_CMA flag
>
> It is a bit of shame that the author of commit d95ea5d1 (happens to be me :)
> was not on cc:.

Sorry about that.
I will add you on cc in next spin. :)

>> >>> 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@mina86.com>
>> >>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> >>>
>> >>> 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.
>>
>> Thanks for giving this a thought for your v2 patch.
>>
>>
>> > If needed, I can implement to track free_area[o].nr_cma_free and use it for
>> > precise freepage calculation in watermark check.
>> >
>> I guess implementing nr_cma_free would be the correct solution.
>> Because currently for other than 0 order allocation
>> we still consider high order free_cma pages as free pages in the for
>> loop which from the code looks incorrect.
>>
>> This can lead to situation when we have more high order free CMA pages
>> but very less unmovable pages, but zone_watermark returns
>> ok for unmovable page, thus leading to allocation failure every time
>> instead of recovering from this situation.
>>
>> But its better if experts comment on this.
>
> I think that implementing free_area[].nr_cma_free is a correct long-term
> solution and it should be done before the current patch gets applied.

Okay.

> [ Tomasz is on holiday currently but he should be back tomorrow so he can
>   also take a look at the issue. ]

Okay.

Thanks.

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