Re: [PATCH v17 7/7] mm: Don't split THP page when syscall is called

[Michal Hocko]

On Mon 20-10-14 19:12:04, Minchan Kim wrote:
> We don't need to split THP page when MADV_FREE syscall is
> called. It could be done when VM decide really frees it so
> we could avoid unnecessary THP split.
> 
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Other than a minor comment below
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  include/linux/huge_mm.h |  4 ++++
>  mm/huge_memory.c        | 35 +++++++++++++++++++++++++++++++++++
>  mm/madvise.c            | 21 ++++++++++++++++++++-
>  mm/rmap.c               |  8 ++++++--
>  mm/vmscan.c             | 28 ++++++++++++++++++----------
>  5 files changed, 83 insertions(+), 13 deletions(-)
> 
[...]
> diff --git a/mm/madvise.c b/mm/madvise.c
> index a21584235bb6..84badee5f46d 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -271,8 +271,26 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  	spinlock_t *ptl;
>  	pte_t *pte, ptent;
>  	struct page *page;
> +	unsigned long next;
> +
> +	next = pmd_addr_end(addr, end);
> +	if (pmd_trans_huge(*pmd)) {
> +		if (next - addr != HPAGE_PMD_SIZE) {
> +#ifdef CONFIG_DEBUG_VM
> +			if (!rwsem_is_locked(&mm->mmap_sem)) {
> +				pr_err("%s: mmap_sem is unlocked! addr=0x%lx end=0x%lx vma->vm_start=0x%lx vma->vm_end=0x%lx\n",
> +					__func__, addr, end,
> +					vma->vm_start,
> +					vma->vm_end);
> +				BUG();
> +			}
> +#endif

Why is this code here? madvise_free_pte_range is called only from the
madvise path and we are holding mmap_sem and relying on that for regular
pages as well.

> +			split_huge_page_pmd(vma, addr, pmd);
> +		} else if (!madvise_free_huge_pmd(tlb, vma, pmd, addr))
> +			goto next;
> +		/* fall through */
> +	}
>  
> -	split_huge_page_pmd(vma, addr, pmd);
>  	if (pmd_trans_unstable(pmd))
>  		return 0;
>  
> @@ -316,6 +334,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  	}
>  	arch_leave_lazy_mmu_mode();
>  	pte_unmap_unlock(pte - 1, ptl);
> +next:
>  	cond_resched();
>  	return 0;
>  }
[...]
-- 
Michal Hocko
SUSE Labs
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