linux-kernel - Re: [PATCH v3 3/4] mm/swap: remove confusing checking for non_swap_entry() in swap_ra

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Date:   Wed, 21 Apr 2021 08:58:04 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <akpm@...ux-foundation.org>, <dennis@...nel.org>,
        <tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
        <hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <willy@...radead.org>, <minchan@...nel.org>,
        <richard.weiyang@...il.com>, <shy828301@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH v3 3/4] mm/swap: remove confusing checking for
 non_swap_entry() in swap_ra_info()

Miaohe Lin <linmiaohe@...wei.com> writes:

> The non_swap_entry() was used for working with VMA based swap readahead
> via commit ec560175c0b6 ("mm, swap: VMA based swap readahead").

At that time, the non_swap_entry() checking is necessary because the
function is called before checking that in do_swap_page().

> Then it's
> moved to swap_ra_info() since commit eaf649ebc3ac ("mm: swap: clean up swap
> readahead").

After that, the non_swap_entry() checking is unnecessary, because
swap_ra_info() is called after non_swap_entry() has been checked
already.  The resulting code is confusing.

> But this makes the code confusing. The non_swap_entry() check
> looks racy because while we released the pte lock, somebody else might have
> faulted in this pte. So we should check whether it's swap pte first to
> guard against such race or swap_type will be unexpected.

The race isn't important because it will not cause problem.

Best Regards,
Huang, Ying

> But the swap_entry
> isn't used in this function and we will have enough checking when we really
> operate the PTE entries later. So checking for non_swap_entry() is not
> really needed here and should be removed to avoid confusion.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/swap_state.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 272ea2108c9d..df5405384520 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -721,7 +721,6 @@ static void swap_ra_info(struct vm_fault *vmf,
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	unsigned long ra_val;
> -	swp_entry_t entry;
>  	unsigned long faddr, pfn, fpfn;
>  	unsigned long start, end;
>  	pte_t *pte, *orig_pte;
> @@ -739,11 +738,6 @@ static void swap_ra_info(struct vm_fault *vmf,
>  
>  	faddr = vmf->address;
>  	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
> -	entry = pte_to_swp_entry(*pte);
> -	if ((unlikely(non_swap_entry(entry)))) {
> -		pte_unmap(orig_pte);
> -		return;
> -	}
>  
>  	fpfn = PFN_DOWN(faddr);
>  	ra_val = GET_SWAP_RA_VAL(vma);