Re: [Patch v2] mm/rmap.c: split huge pmd when it really is

[Wei Yang]

On Tue, Dec 24, 2019 at 06:28:56AM +0800, Wei Yang wrote:
>When page is not NULL, function is called by try_to_unmap_one() with
>TTU_SPLIT_HUGE_PMD set. There are two cases to call try_to_unmap_one()
>with TTU_SPLIT_HUGE_PMD set:
>
>  * unmap_page()
>  * shrink_page_list()
>
>In both case, the page passed to try_to_unmap_one() is PageHead() of the
>THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
>aligned, this means the THP is not mapped as PMD THP in this process.
>This could happen when we do mremap() a PMD size range to an un-aligned
>address.
>
>Currently, this case is handled by following check in __split_huge_pmd()
>luckily.
>
>  page != pmd_page(*pmd)
>
>This patch checks the address to skip some work.

I am sorry to forget address Kirill's comment in 1st version.

The first one is the performance difference after this change for a PTE
mappged THP.

Here is the result:(in cycle)

        Before     Patched

        963        195
        988        40
        895        78

Average 948        104

So the change reduced 90% time for function split_huge_pmd_address().

For the 2nd comment, the vma check. Let me take a further look to analysis.

Thanks for Kirill's suggestion.

>
>Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
>
>---
>v2: move the check into split_huge_pmd_address().
>---
> mm/huge_memory.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 893fecd5daa4..2b9c2f412b32 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2342,6 +2342,22 @@ void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
> 	pud_t *pud;
> 	pmd_t *pmd;
> 
>+	/*
>+	 * When page is not NULL, function is called by try_to_unmap_one()
>+	 * with TTU_SPLIT_HUGE_PMD set. There are two places set
>+	 * TTU_SPLIT_HUGE_PMD
>+	 *
>+	 *     unmap_page()
>+	 *     shrink_page_list()
>+	 *
>+	 * In both cases, the "page" here is the PageHead() of a THP.
>+	 *
>+	 * If the page is not a PMD mapped huge page, e.g. after mremap(), it
>+	 * is not necessary to split it.
>+	 */
>+	if (page && !IS_ALIGNED(address, HPAGE_PMD_SIZE))
>+		return;
>+
> 	pgd = pgd_offset(vma->vm_mm, address);
> 	if (!pgd_present(*pgd))
> 		return;
>-- 
>2.17.1

-- 
Wei Yang
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