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Date:   Tue, 15 Sep 2020 17:03:49 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Jann Horn <jannh@google.com>, Jan Kara <jack@suse.cz>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Message-ID: <20200915150349.GA22415@redhat.com>
References: <20200821234958.7896-1-peterx@redhat.com>
 <20200821234958.7896-2-peterx@redhat.com>
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On 08/21, Peter Xu wrote:
>
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2927,50 +2927,25 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  	 * not dirty accountable.
>  	 */
>  	if (PageAnon(vmf->page)) {
> -		int total_map_swapcount;
> -		if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
> -					   page_count(vmf->page) != 1))
> +		struct page *page = vmf->page;
> +
> +		/* PageKsm() doesn't necessarily raise the page refcount */
> +		if (PageKsm(page) || page_count(page) != 1)
> +			goto copy;
> +		if (!trylock_page(page))
> +			goto copy;
> +		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
> +			unlock_page(page);
>  			goto copy;
> -		if (!trylock_page(vmf->page)) {
> -			get_page(vmf->page);
> -			pte_unmap_unlock(vmf->pte, vmf->ptl);
> -			lock_page(vmf->page);
> -			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> -					vmf->address, &vmf->ptl);
> -			if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> -				update_mmu_tlb(vma, vmf->address, vmf->pte);
> -				unlock_page(vmf->page);
> -				pte_unmap_unlock(vmf->pte, vmf->ptl);
> -				put_page(vmf->page);
> -				return 0;
> -			}
> -			put_page(vmf->page);
> -		}
> -		if (PageKsm(vmf->page)) {
> -			bool reused = reuse_ksm_page(vmf->page, vmf->vma,
> -						     vmf->address);
> -			unlock_page(vmf->page);
> -			if (!reused)
> -				goto copy;
> -			wp_page_reuse(vmf);
> -			return VM_FAULT_WRITE;
> -		}
> -		if (reuse_swap_page(vmf->page, &total_map_swapcount)) {

It seems that nobody else calls reuse_swap_page() with total_map_swapcount != NULL.

Oleg.