linux-kernel - Re: [PATCH 2/7] mm/khugepaged: stop swapping in page when VM_FAULT

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Date:   Wed, 15 Jun 2022 08:14:32 -0700
From:   Zach O'Keefe <zokeefe@...gle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, aarcange@...hat.com,
        willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, david@...hat.com,
        surenb@...gle.com, peterx@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/7] mm/khugepaged: stop swapping in page when
 VM_FAULT_RETRY occurs

On 11 Jun 16:47, Miaohe Lin wrote:
> When do_swap_page returns VM_FAULT_RETRY, we do not retry here and thus
> swap entry will remain in pagetable. This will result in later failure.
> So stop swapping in pages in this case to save cpu cycles.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/khugepaged.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 73570dfffcec..a8adb2d1e9c6 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1003,19 +1003,16 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>  		swapped_in++;
>  		ret = do_swap_page(&vmf);
>  
> -		/* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
> +		/*
> +		 * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
> +		 * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
> +		 * we do not retry here and swap entry will remain in pagetable
> +		 * resulting in later failure.
> +		 */
>  		if (ret & VM_FAULT_RETRY) {
>  			mmap_read_lock(mm);
> -			if (hugepage_vma_revalidate(mm, haddr, &vma)) {
> -				/* vma is no longer available, don't continue to swapin */
> -				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> -				return false;
> -			}
> -			/* check if the pmd is still valid */
> -			if (mm_find_pmd(mm, haddr) != pmd) {
> -				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> -				return false;
> -			}
> +			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> +			return false;
>  		}
>  		if (ret & VM_FAULT_ERROR) {
>  			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> -- 
> 2.23.0
> 
>

I've convinced myself this is correct, but don't understand how we got here.
AFAICT, we've always continued to fault in pages, and, as you mention, don't
retry ones that have failed with VM_FAULT_RETRY - so
__collapse_huge_page_isolate() should fail. I don't think (?) there is any
benefit to continuing to swap if we don't handle VM_FAULT_RETRY appropriately.

So, I think this change looks good from that perspective. I suppose the only
other question would be: should we handle the VM_FAULT_RETRY case? Maybe 1
additional attempt then fail? AFAIK, this mostly (?) happens when the page is
locked.  Maybe it's not worth the extra complexity though..