linux-kernel - Re: [PATCH v3 6/9] mm/khugepaged: remove reuse_swap

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Date:   Thu, 10 Mar 2022 11:37:36 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 6/9] mm/khugepaged: remove reuse_swap_page() usage

On 1/31/22 17:29, David Hildenbrand wrote:
> reuse_swap_page() currently indicates if we can write to an anon page
> without COW. A COW is required if the page is shared by multiple
> processes (either already mapped or via swap entries) or if there is
> concurrent writeback that cannot tolerate concurrent page modifications.
> 
> However, in the context of khugepaged we're not actually going to write
> to a read-only mapped page, we'll copy the page content to our newly
> allocated THP and map that THP writable. All we have to make sure
> is that the read-only mapped page we're about to copy won't get reused
> by another process sharing the page, otherwise, page content would
> get modified. But that is already guaranteed via multiple mechanisms
> (e.g., holding a reference, holding the page lock, removing the rmap after
>  copying the page).
> 
> The swapcache handling was introduced in commit 10359213d05a ("mm:
> incorporate read-only pages into transparent huge pages") and it sounds
> like it merely wanted to mimic what do_swap_page() would do when trying
> to map a page obtained via the swapcache writable.
> 
> As that logic is unnecessary, let's just remove it, removing the last
> user of reuse_swap_page().
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/trace/events/huge_memory.h |  1 -
>  mm/khugepaged.c                    | 11 -----------
>  2 files changed, 12 deletions(-)
> 
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 4fdb14a81108..d651f3437367 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -29,7 +29,6 @@
>  	EM( SCAN_VMA_NULL,		"vma_null")			\
>  	EM( SCAN_VMA_CHECK,		"vma_check_failed")		\
>  	EM( SCAN_ADDRESS_RANGE,		"not_suitable_address_range")	\
> -	EM( SCAN_SWAP_CACHE_PAGE,	"page_swap_cache")		\
>  	EM( SCAN_DEL_PAGE_LRU,		"could_not_delete_page_from_lru")\
>  	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
>  	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 35f14d0a00a6..9da9325ab4d4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -45,7 +45,6 @@ enum scan_result {
>  	SCAN_VMA_NULL,
>  	SCAN_VMA_CHECK,
>  	SCAN_ADDRESS_RANGE,
> -	SCAN_SWAP_CACHE_PAGE,
>  	SCAN_DEL_PAGE_LRU,
>  	SCAN_ALLOC_HUGE_PAGE_FAIL,
>  	SCAN_CGROUP_CHARGE_FAIL,
> @@ -682,16 +681,6 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  			result = SCAN_PAGE_COUNT;
>  			goto out;
>  		}
> -		if (!pte_write(pteval) && PageSwapCache(page) &&
> -				!reuse_swap_page(page)) {
> -			/*
> -			 * Page is in the swap cache and cannot be re-used.
> -			 * It cannot be collapsed into a THP.
> -			 */
> -			unlock_page(page);
> -			result = SCAN_SWAP_CACHE_PAGE;
> -			goto out;
> -		}
>  
>  		/*
>  		 * Isolate the page to avoid collapsing an hugepage