Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 9 Jun 2023 13:11:18 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Andrew Morton <akpm@linux-foundation.org>
cc:     Hugh Dickins <hughd@google.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Huang Ying <ying.huang@intel.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        Ryan Roberts <ryan.roberts@arm.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2 28/32] mm/memory: allow pte_offset_map[_lock]() to
 fail
In-Reply-To: <20230609130632.ec6ffe72fc5f7952af4a3e54@linux-foundation.org>
Message-ID: <11a9744a-e7f-33d0-474-c2f2eb7e079@google.com>
References: <c1c9a74a-bc5b-15ea-e5d2-8ec34bc921d@google.com> <bb548d50-e99a-f29e-eab1-a43bef2a1287@google.com> <20230609130632.ec6ffe72fc5f7952af4a3e54@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Fri, 9 Jun 2023, Andrew Morton wrote:
> On Thu, 8 Jun 2023 18:43:38 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
> 
> > copy_pte_range(): use pte_offset_map_nolock(), and allow for it to fail;
> > but with a comment on some further assumptions that are being made there.
> > 
> > zap_pte_range() and zap_pmd_range(): adjust their interaction so that
> > a pte_offset_map_lock() failure in zap_pte_range() leads to a retry in
> > zap_pmd_range(); remove call to pmd_none_or_trans_huge_or_clear_bad().
> > 
> > Allow pte_offset_map_lock() to fail in many functions.  Update comment
> > on calling pte_alloc() in do_anonymous_page().  Remove redundant calls
> > to pmd_trans_unstable(), pmd_devmap_trans_unstable(), pmd_none() and
> > pmd_bad(); but leave pmd_none_or_clear_bad() calls in free_pmd_range()
> > and copy_pmd_range(), those do simplify the next level down.
> > 
> > ...
> >
> > @@ -3728,11 +3737,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >  			vmf->page = pfn_swap_entry_to_page(entry);
> >  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> >  					vmf->address, &vmf->ptl);
> > -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {
> > -				spin_unlock(vmf->ptl);
> > -				goto out;
> > -			}
> > -
> > +			if (unlikely(!vmf->pte ||
> > +				     !pte_same(*vmf->pte, vmf->orig_pte)))
> > +				goto unlock;
> >  			/*
> >  			 * Get a page reference while we know the page can't be
> >  			 * freed.
> 
> This hunk falls afoul of
> https://lkml.kernel.org/r/20230602092949.545577-5-ryan.roberts@arm.com.
> 
> I did this:
> 
> @@ -3729,7 +3738,8 @@ vm_fault_t do_swap_page(struct vm_fault
>  			vmf->page = pfn_swap_entry_to_page(entry);
>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>  					vmf->address, &vmf->ptl);
> -			if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte)))
> +			if (unlikely(!vmf->pte ||
> +				     !pte_same(*vmf->pte, vmf->orig_pte)))
>  				goto unlock;
>  
>  			/*

Yes, that's exactly right: thanks, Andrew.

Hugh