linux-kernel - Re: [PATCH 4/8] mm/memory-failure.c: remove unneeded orig

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Date:   Mon, 14 Feb 2022 23:50:19 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] mm/memory-failure.c: remove unneeded orig_head

On Thu, Feb 10, 2022 at 10:17:29PM +0800, Miaohe Lin wrote:
> orig_head is used to check whether the page have changed compound pages
> during the locking. But it's always equal to hpage. So we can use hpage
> directly and remove this redundant one.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/memory-failure.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 2dd7f35ee65a..4370c2f407c5 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1691,7 +1691,6 @@ int memory_failure(unsigned long pfn, int flags)
>  {
>  	struct page *p;
>  	struct page *hpage;
> -	struct page *orig_head;
>  	struct dev_pagemap *pgmap;
>  	int res = 0;
>  	unsigned long page_flags;
> @@ -1737,7 +1736,7 @@ int memory_failure(unsigned long pfn, int flags)
>  		goto unlock_mutex;
>  	}
> 
> -	orig_head = hpage = compound_head(p);
> +	hpage = compound_head(p);
>  	num_poisoned_pages_inc();
> 
>  	/*
> @@ -1821,7 +1820,7 @@ int memory_failure(unsigned long pfn, int flags)
>  	 * The page could have changed compound pages during the locking.
>  	 * If this happens just bail out.
>  	 */
> -	if (PageCompound(p) && compound_head(p) != orig_head) {
> +	if (PageCompound(p) && compound_head(p) != hpage) {

I think that this if-check was intended to detect the case that page p
belongs to a thp when memory_failure() is called and belongs to a compound
page in different size (like slab or some driver page) after the thp is
split.  But your suggestion makes me aware that the page p could be embedded
on a thp again after thp split.  I think this might be rare, but if it
happens the current if-check (or suggested one) cannot detect it.
So I feel that simply dropping compound_head() check might be better?

-	if (PageCompound(p) && compound_head(p) != orig_head) {
+	if (PageCompound(p)) {

This should ensure the assumption (mentioned in 8/8 patch) more that
the error page never belongs to compound page after taking page lock.

Thanks,
Naoya Horiguchi