Date:   Mon, 20 Dec 2021 18:52:59 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Nadav Amit <namit@vmware.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On Mon, Dec 20, 2021 at 06:37:30PM +0000, Matthew Wilcox wrote:
> +++ b/mm/memory.c
> @@ -3626,7 +3626,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
>  	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
>  	pte = mk_pte(page, vma->vm_page_prot);
> -	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
> +	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
>  		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
>  		vmf->flags &= ~FAULT_FLAG_WRITE;
>  		ret |= VM_FAULT_WRITE;
[...]
> @@ -1673,17 +1665,14 @@ static int page_trans_huge_map_swapcount(struct page *page, int *total_mapcount,
>   * reuse_swap_page() returns false, but it may be always overwritten
>   * (see the other implementation for CONFIG_SWAP=n).
>   */
> -bool reuse_swap_page(struct page *page, int *total_map_swapcount)
> +bool reuse_swap_page(struct page *page)
>  {
> -	int count, total_mapcount, total_swapcount;
> +	int count, total_swapcount;
>  
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	if (unlikely(PageKsm(page)))
>  		return false;
> -	count = page_trans_huge_map_swapcount(page, &total_mapcount,
> -					      &total_swapcount);
> -	if (total_map_swapcount)
> -		*total_map_swapcount = total_mapcount + total_swapcount;
> +	count = page_trans_huge_map_swapcount(page, &total_swapcount);
>  	if (count == 1 && PageSwapCache(page) &&
>  	    (likely(!PageTransCompound(page)) ||
>  	     /* The remaining swap count will be freed soon */

It makes me wonder if reuse_swap_page() can also be based on refcount
instead of mapcount?