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

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Date:   Mon, 26 Oct 2020 02:46:09 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.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

Andrew Morton <akpm@...ux-foundation.org> wrote:
> 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.
>> 
>
>Looks right.  But I think it would be better to fix this in
>swap_ra_info() - make it more consistent in its return state.
>

I think modernify the struct initializer is better because it means we have to swap in one page at least.
Many thanks.

>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)
>_
>