linux-kernel - Re: [PATCH] mm/swap_state: Skip meaningless swap cache readahead when ra

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Message-Id: <20201025165948.bd4f59d127952b9979d5c61e@linux-foundation.org>
Date:   Sun, 25 Oct 2020 16:59:48 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm/swap_state: Skip meaningless swap cache readahead
 when ra_info.win == 0

On Fri, 9 Oct 2020 09:30:59 -0400 Miaohe Lin <linmiaohe@...wei.com> wrote:

> swap_ra_info() may leave ra_info untouched in non_swap_entry() case as page
> table lock is not held. In this case, we have ra_info.nr_pte == 0 and it is
> meaningless to continue with swap cache readahead. Skip such ops by init
> ra_info.win = 1.
> 
>
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -839,7 +839,7 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
>  	swp_entry_t entry;
>  	unsigned int i;
>  	bool page_allocated;
> -	struct vma_swap_readahead ra_info = {0,};
> +	struct vma_swap_readahead ra_info = {1,};
>  
>  	swap_ra_info(vmf, &ra_info);
>  	if (ra_info.win == 1)

Looks right.  But I think it would be better to fix this in
swap_ra_info() - make it more consistent in its return state.

Otherwise, let's modernify that struct initializer:

--- a/mm/swap_state.c~mm-swap_state-skip-meaningless-swap-cache-readahead-when-ra_infowin-==-0-fix
+++ a/mm/swap_state.c
@@ -839,7 +839,9 @@ static struct page *swap_vma_readahead(s
 	swp_entry_t entry;
 	unsigned int i;
 	bool page_allocated;
-	struct vma_swap_readahead ra_info = {1,};
+	struct vma_swap_readahead ra_info = {
+		.win = 1,
+	};
 
 	swap_ra_info(vmf, &ra_info);
 	if (ra_info.win == 1)
_