Re: [PATCH 3/3] mm, thp: make swapin readahead under down_read of mmap_sem

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, May 23, 2016 at 08:14:11PM +0300, Ebru Akagunduz wrote:
> Currently khugepaged makes swapin readahead under
> down_write. This patch supplies to make swapin
> readahead under down_read instead of down_write.
> 
> The patch was tested with a test program that allocates
> 800MB of memory, writes to it, and then sleeps. The system
> was forced to swap out all. Afterwards, the test program
> touches the area by writing, it skips a page in each
> 20 pages of the area.
> 
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> ---
>  mm/huge_memory.c | 33 +++++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index feee44c..668bc07 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2386,13 +2386,14 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
>   * but with mmap_sem held to protect against vma changes.
>   */
>  
> -static void __collapse_huge_page_swapin(struct mm_struct *mm,
> +static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>  					struct vm_area_struct *vma,
>  					unsigned long address, pmd_t *pmd)
>  {
>  	unsigned long _address;
>  	pte_t *pte, pteval;
>  	int swapped_in = 0, ret = 0;
> +	struct vm_area_struct *vma_orig = vma;
>  
>  	pte = pte_offset_map(pmd, address);
>  	for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
> @@ -2402,11 +2403,19 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
>  			continue;
>  		swapped_in++;
>  		ret = do_swap_page(mm, vma, _address, pte, pmd,
> -				   FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
> +				   FAULT_FLAG_ALLOW_RETRY,
>  				   pteval);
> +		/* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
> +		if (ret & VM_FAULT_RETRY) {
> +			down_read(&mm->mmap_sem);
> +			vma = find_vma(mm, address);
> +			/* vma is no longer available, don't continue to swapin */
> +			if (vma != vma_orig)
> +				return false;
> +		}
>  		if (ret & VM_FAULT_ERROR) {
>  			trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
> -			return;
> +			return false;
>  		}
>  		/* pte is unmapped now, we need to map it */
>  		pte = pte_offset_map(pmd, _address);
> @@ -2414,6 +2423,7 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
>  	pte--;
>  	pte_unmap(pte);
>  	trace_mm_collapse_huge_page_swapin(mm, swapped_in, 1);
> +	return true;
>  }
>  
>  static void collapse_huge_page(struct mm_struct *mm,
> @@ -2459,7 +2469,7 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	 * gup_fast later hanlded by the ptep_clear_flush and the VM
>  	 * handled by the anon_vma lock + PG_lock.
>  	 */
> -	down_write(&mm->mmap_sem);
> +	down_read(&mm->mmap_sem);
>  	if (unlikely(khugepaged_test_exit(mm))) {
>  		result = SCAN_ANY_PROCESS;
>  		goto out;
> @@ -2490,9 +2500,20 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	 * Don't perform swapin readahead when the system is under pressure,
>  	 * to avoid unnecessary resource consumption.
>  	 */
> -	if (allocstall == curr_allocstall && swap != 0)
> -		__collapse_huge_page_swapin(mm, vma, address, pmd);
> +	if (allocstall == curr_allocstall && swap != 0) {
> +		/*
> +		 * __collapse_huge_page_swapin always returns with mmap_sem
> +		 * locked. If it fails, release mmap_sem and jump directly
> +		 * label out. Continuing to collapse causes inconsistency.
> +		 */
> +		if (!__collapse_huge_page_swapin(mm, vma, address, pmd)) {
> +			up_read(&mm->mmap_sem);
> +			goto out;
> +		}
> +	}
>  
> +	up_read(&mm->mmap_sem);
> +	down_write(&mm->mmap_sem);

That's the critical point.

How do you guarantee that the vma will not be destroyed (or changed)
between up_read() and down_write()?

You need at least find_vma() again.

>  	anon_vma_lock_write(vma->anon_vma);
>  
>  	pte = pte_offset_map(pmd, address);
> -- 
> 1.9.1
> 
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-- 
 Kirill A. Shutemov

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