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[2003:d8:2f28:4600:d3a7:6c26:54cf:e01e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42100f1399asm141833865e9.11.2024.05.27.06.17.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 May 2024 06:17:02 -0700 (PDT) Message-ID: <9f3371fa-0d46-4e93-a00b-0f5cb30b58bb@redhat.com> Date: Mon, 27 May 2024 15:17:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: Move vmf_anon_prepare upfront in hugetlb_wp To: Oscar Salvador , Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , Vishal Moola References: <20240521073446.23185-1-osalvador@suse.de> From: David Hildenbrand In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 8A94240007 X-Stat-Signature: um8xdqn6en44ch7kpsjhgypjutr6h3aq X-Rspam-User: X-HE-Tag: 1716815827-456020 X-HE-Meta: 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 +JhTIuhj Uz/vuEuQk17OqUxWJxGeapqdBH0UoAR3fy+miHAlypeTtcPUNPoEgXmB1aFhsZ4w57tjgZcpEHcX11t9Ag4iFtbJP8IWhwpPvHLPap3c55rcL5wNSo+mFx/Fy87M6cIcQxgMu8WEcp1nprlaGxu99Jfge24PAbAqjD46SXqc4zXKDjGT3LNOabLuEAeIiK94/QdzA+H/0i49mul9ZLa3cUXV0PBjLtnlZ9BPxn2hBYVGXPFZ6Cp+wgd2MeY4X8bLDRIAhYa35vbBWeKhOJb9h1I4kY9ZEmdrmfaAC+hB5FHn9Jvyc4y3bWcnxp1aqe0HSMcsI3+EFqDMbtJbqirbjZzdp/OE1iWSdCYQCylP+rKYygf/ZI1UvlO9vuueSCq7e+bSq+18vQDW9Y70eufNV3+ayXLo4y5uj+61EFQc9spr2cOzy/uGv1jP6cDNtTS8dj81eZ0XwnUhlDMbwNlQK3tzcPEhLYEEtgX+b+xKrjVMVCphz6ogw71Dh6uclQzf78cQEMnbcc2x5bL3M9P5c+Q5sE1bqoG08+pI83Rcgo0JNaGY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.284363, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Am 27.05.24 um 10:53 schrieb Oscar Salvador: > On Tue, May 21, 2024 at 09:34:46AM +0200, Oscar Salvador wrote: >> I did not hit this bug, I just spotted this because I was looking at hugetlb_wp >> for some other reason. And I did not want to get creative to see if I could >> trigger this so I could get a backtrace. >> My assumption is that we could trigger this if 1) this was a shared mapping, >> so no anon_vma and 2) we call in GUP code with FOLL_WRITE, which would cause >> the FLAG_UNSHARE to be passed, so we will end up in hugetlb_wp(). > > So I checked this again and I have to confess I am bit confused. > > hugetlb_wp() can be called from either hugetlb_fault() or hugetlb_no_page(). > > hugetlb_fault()->hugetlb_wp() upon FAULT_FLAG_{WRITE,UNSHARE} > hugetlb_no_page->hugetlb_wp()-> upon FAULT_FLAG_WRITE && !VM_SHARED > > hugetlb_no_page()->vmf_anon_prepare() upon !VM_SHARED, which means that VM_SHARED > mappings do not have vma->anon_vma, while others do. > > hugetlb_wp() will call set_huge_ptep_writable() right away and return if it sees > that the mapping is shared. > So the only other we have to end up in hugetlb_wp() is via FAULT_FLAG_UNSHARE. > For that to happen gup_must_unshare() must return true, which means the following > assumptions must hold. > > - For Anonymous pages: > 1) !PageAnonExclusive > - For Filebacked pages: > 2) We do not have a vma > 3) It is a COW mapping > > 1) If gup_must_unshare() returns true for Anonymous pages because the page is not > exclusive and must be unshared, hugetlb_wp() will already see the > vma->anon_prepare being initialized because of the previous > hugetlb_no_page()->vmf_anon_prepare. > > 2) I do not quite understand this case. gup_must_unshare() without a VMA is only used for GUP-fast. Before triggering a page fault we always fallback to GUP-slow first, where we have a VMA. IMHO we better not make assumptions that hugetlb_wp() will always already have an anon VMA. -- Thanks, David / dhildenb