Re: [RFC/PATCH 4/5] mm: get_user_pages: migrate out CMA pages when FOLL_DURABLE flag is set

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

2013/03/05 15:57, Marek Szyprowski wrote:
> When __get_user_pages() is called with FOLL_DURABLE flag, ensure that no
> page in CMA pageblocks gets locked. This workarounds the permanent
> migration failures caused by locking the pages by get_user_pages() call for
> a long period of time.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> ---
>   mm/internal.h |   12 ++++++++++++
>   mm/memory.c   |   43 +++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 55 insertions(+)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 8562de0..a290d04 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -105,6 +105,18 @@ extern void prep_compound_page(struct page *page, unsigned long order);
>   extern bool is_free_buddy_page(struct page *page);
>   #endif
>   
> +#ifdef CONFIG_CMA
> +static inline int is_cma_page(struct page *page)
> +{
> +	unsigned mt = get_pageblock_migratetype(page);
> +	if (mt == MIGRATE_ISOLATE || mt == MIGRATE_CMA)
> +		return true;
> +	return false;
> +}
> +#else
> +#define is_cma_page(page) 0
> +#endif
> +
>   #if defined CONFIG_COMPACTION || defined CONFIG_CMA
>   
>   /*
> diff --git a/mm/memory.c b/mm/memory.c
> index 2b9c2dd..f81b273 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1650,6 +1650,45 @@ static inline int stack_guard_page(struct vm_area_struct *vma, unsigned long add
>   }
>   
>   /**
> + * replace_cma_page() - migrate page out of CMA page blocks
> + * @page:	source page to be migrated
> + *
> + * Returns either the old page (if migration was not possible) or the pointer
> + * to the newly allocated page (with additional reference taken).
> + *
> + * get_user_pages() might take a reference to a page for a long period of time,
> + * what prevent such page from migration. This is fatal to the preffered usage
> + * pattern of CMA pageblocks. This function replaces the given user page with
> + * a new one allocated from NON-MOVABLE pageblock, so locking CMA page can be
> + * avoided.
> + */
> +static inline struct page *migrate_replace_cma_page(struct page *page)
> +{
> +	struct page *newpage = alloc_page(GFP_HIGHUSER);
> +
> +	if (!newpage)
> +		goto out;
> +
> +	/*
> +	 * Take additional reference to the new page to ensure it won't get
> +	 * freed after migration procedure end.
> +	 */
> +	get_page_foll(newpage);
> +
> +	if (migrate_replace_page(page, newpage) == 0)
> +		return newpage;
> +
> +	put_page(newpage);
> +	__free_page(newpage);
> +out:
> +	/*
> +	 * Migration errors in case of get_user_pages() might not
> +	 * be fatal to CMA itself, so better don't fail here.
> +	 */
> +	return page;
> +}
> +
> +/**
>    * __get_user_pages() - pin user pages in memory
>    * @tsk:	task_struct of target task
>    * @mm:		mm_struct of target mm
> @@ -1884,6 +1923,10 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>   			}
>   			if (IS_ERR(page))
>   				return i ? i : PTR_ERR(page);

> +
> +			if ((gup_flags & FOLL_DURABLE) && is_cma_page(page))
> +				page = migrate_replace_cma_page(page);
> +

I might be misreading. 
If FOLL_DURABLE is set, this page is always allocated as non movable.
Is it right? If so, when does this situation occur?

Thanks,
Yasuaki Ishimatsu

>   			if (pages) {
>   				pages[i] = page;
>   
> 


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