Re: [PATCH] CMA: Fix CMA's page number is substructed twice in __zone_watermark_ok

From: Hui Zhu
Date: Wed Jan 07 2015 - 04:46:43 EST

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In reply to: Vlastimil Babka: "Re: [PATCH] CMA: Fix CMA's page number is substructed twice in __zone_watermark_ok"
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On Wed, Jan 7, 2015 at 4:45 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 12/30/2014 11:17 AM, Hui Zhu wrote:
>> The original of this patch [1] is used to fix the issue in Joonsoo's CMA patch
>> "CMA: always treat free cma pages as non-free on watermark checking" [2].
>>
>> Joonsoo reminded me that this issue affect current kernel too. So made a new
>> one for upstream.
>>
>> Function __zone_watermark_ok substruct CMA pages number from free_pages
>> if system allocation can't use CMA areas:
>> /* 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);
>>
>> But after this part of code
>> for (o = 0; o < order; o++) {
>> /* At the next order, this order's pages become unavailable */
>> free_pages -= z->free_area[o].nr_free << o;
>> CMA memory in each order is part of z->free_area[o].nr_free, then the CMA
>> page number of this order is substructed twice. This bug will make
>> __zone_watermark_ok return more false.
>>
>> This patch add cma_free_area to struct free_area that just record the number
>> of CMA pages. And add it back in the order loop to handle the substruct
>> twice issue.
>
> Le sigh.
>
> I now dub CMA "Contagious Memory Allocator".
> One can't even take a Christmas vacation without this blight to spread :(
>
> Seriously, with so much special casing everywhere in fast paths, Minchan's
> (IIRC) proposal of a special CMA zone has some appeal.
>
> But it seems to me that the bug you are fixing doesn't exist as you describe it?
> free_cma is only used here:
>
> if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> return false;
>
> So it's subtracted from free_pages just temporarily for the basic order-0 check.
> In the higher-order magic loop, it's not used at all?
>

I am so sorry that I made a mistake when I split this patch from the
patch series.

The original of this patch is to fix the issue around Joonsoo's update
of __zone_watermark_ok:
if (IS_ENABLED(CONFIG_CMA) && z->managed_cma_pages)
free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);

if (free_pages <= min + z->lowmem_reserve[classzone_idx])
return false;

Joonsoo, what about submit this change to upstream first?

Thanks,
Hui

> Vlastimil
>
>
>> [1] https://lkml.org/lkml/2014/12/25/43
>> [2] https://lkml.org/lkml/2014/5/28/110
>>
>> Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
>> Signed-off-by: Weixing Liu <liuweixing@xxxxxxxxxx>
>> ---
>> include/linux/mmzone.h | 3 +++
>> mm/page_alloc.c | 22 ++++++++++++++++++++++
>> 2 files changed, 25 insertions(+)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 2f0856d..094476b 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -92,6 +92,9 @@ static inline int get_pfnblock_migratetype(struct page *page, unsigned long pfn)
>> struct free_area {
>> struct list_head free_list[MIGRATE_TYPES];
>> unsigned long nr_free;
>> +#ifdef CONFIG_CMA
>> + unsigned long cma_nr_free;
>> +#endif
>> };
>>
>> struct pglist_data;
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 7633c50..026cf27 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -650,6 +650,8 @@ static inline void __free_one_page(struct page *page,
>> } else {
>> list_del(&buddy->lru);
>> zone->free_area[order].nr_free--;
>> + if (is_migrate_cma(migratetype))
>> + zone->free_area[order].cma_nr_free--;
>> rmv_page_order(buddy);
>> }
>> combined_idx = buddy_idx & page_idx;
>> @@ -683,6 +685,8 @@ static inline void __free_one_page(struct page *page,
>> list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
>> out:
>> zone->free_area[order].nr_free++;
>> + if (is_migrate_cma(migratetype))
>> + zone->free_area[order].cma_nr_free++;
>> }
>>
>> static inline int free_pages_check(struct page *page)
>> @@ -937,6 +941,8 @@ static inline void expand(struct zone *zone, struct page *page,
>> }
>> list_add(&page[size].lru, &area->free_list[migratetype]);
>> area->nr_free++;
>> + if (is_migrate_cma(migratetype))
>> + area->cma_nr_free++;
>> set_page_order(&page[size], high);
>> }
>> }
>> @@ -1020,6 +1026,8 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
>> list_del(&page->lru);
>> rmv_page_order(page);
>> area->nr_free--;
>> + if (is_migrate_cma(migratetype))
>> + area->cma_nr_free--;
>> expand(zone, page, order, current_order, area, migratetype);
>> set_freepage_migratetype(page, migratetype);
>> return page;
>> @@ -1208,6 +1216,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype)
>> page = list_entry(area->free_list[migratetype].next,
>> struct page, lru);
>> area->nr_free--;
>> + if (is_migrate_cma(migratetype))
>> + area->cma_nr_free--;
>>
>> new_type = try_to_steal_freepages(zone, page,
>> start_migratetype,
>> @@ -1597,6 +1607,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
>> /* Remove page from free list */
>> list_del(&page->lru);
>> zone->free_area[order].nr_free--;
>> + if (is_migrate_cma(mt))
>> + zone->free_area[order].cma_nr_free--;
>> rmv_page_order(page);
>>
>> /* Set the pageblock if the isolated page is at least a pageblock */
>> @@ -1827,6 +1839,13 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
>> /* At the next order, this order's pages become unavailable */
>> free_pages -= z->free_area[o].nr_free << o;
>>
>> + /* If CMA's page number of this order was substructed as part
>> + of "zone_page_state(z, NR_FREE_CMA_PAGES)", subtracting
>> + "z->free_area[o].nr_free << o" substructed CMA's page
>> + number of this order again. So add it back. */
>> + if (IS_ENABLED(CONFIG_CMA) && free_cma)
>> + free_pages += z->free_area[o].cma_nr_free << o;
>> +
>> /* Require fewer higher order pages to be free */
>> min >>= 1;
>>
>> @@ -4238,6 +4257,7 @@ static void __meminit zone_init_free_lists(struct zone *zone)
>> for_each_migratetype_order(order, t) {
>> INIT_LIST_HEAD(&zone->free_area[order].free_list[t]);
>> zone->free_area[order].nr_free = 0;
>> + zone->free_area[order].cma_nr_free = 0;
>> }
>> }
>>
>> @@ -6610,6 +6630,8 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
>> list_del(&page->lru);
>> rmv_page_order(page);
>> zone->free_area[order].nr_free--;
>> + if (is_migrate_cma(get_pageblock_migratetype(page)))
>> + zone->free_area[order].cma_nr_free--;
>> for (i = 0; i < (1 << order); i++)
>> SetPageReserved((page+i));
>> pfn += (1 << order);
>>
>
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Next message: James Hogan: "Re: [PATCH v2 09/40] metag/uaccess: fix sparse errors"
Previous message: Fabian Holler: "[PATCH 2/3] staging: android: remove blank lines after open braces"
In reply to: Vlastimil Babka: "Re: [PATCH] CMA: Fix CMA's page number is substructed twice in __zone_watermark_ok"
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