Re: [PATCHv5 26/28] thp: introduce deferred_split_huge_page()

[Vlastimil Babka]

On 04/23/2015 11:04 PM, Kirill A. Shutemov wrote:
> Currently we don't split huge page on partial unmap. It's not an ideal
> situation. It can lead to memory overhead.
>
> Furtunately, we can detect partial unmap on page_remove_rmap(). But we
> cannot call split_huge_page() from there due to locking context.
>
> It's also counterproductive to do directly from munmap() codepath: in
> many cases we will hit this from exit(2) and splitting the huge page
> just to free it up in small pages is not what we really want.
>
> The patch introduce deferred_split_huge_page() which put the huge page
> into queue for splitting. The splitting itself will happen when we get
> memory pressure via shrinker interface. The page will be dropped from
> list on freeing through compound page destructor.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> @@ -715,6 +726,12 @@ static inline pmd_t mk_huge_pmd(struct page *page, pgprot_t prot)
>   	return entry;
>   }
>
> +void prep_transhuge_page(struct page *page)
> +{
> +	INIT_LIST_HEAD(&page[2].lru);

Wouldn't hurt to mention that you use page[2] because lru in page 1 
would collide with the dtor (right?).

> +	set_compound_page_dtor(page, free_transhuge_page);
> +}
> +
>   static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
>   					struct vm_area_struct *vma,
>   					unsigned long haddr, pmd_t *pmd,

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