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[2003:cb:c708:bc00:2acb:9e46:1412:686a]) by smtp.gmail.com with ESMTPSA id j1-20020a05600c410100b003dec22de1b1sm1096006wmi.10.2023.02.16.01.37.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Feb 2023 01:37:37 -0800 (PST) Message-ID: Date: Thu, 16 Feb 2023 10:37:36 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 To: Muhammad Usama Anjum , peterx@redhat.com, Andrew Morton Cc: kernel@collabora.com, Paul Gofman , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20230216091656.2045471-1-usama.anjum@collabora.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v4 1/2] mm/userfaultfd: Support WP on multiple VMAs In-Reply-To: <20230216091656.2045471-1-usama.anjum@collabora.com> 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: rspam07 X-Rspamd-Queue-Id: 0B63E20012 X-Rspam-User: X-Stat-Signature: nsa8jxftp6h8rbkk5466eon56musxupr X-HE-Tag: 1676540260-635449 X-HE-Meta: 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 Wbm2mFHC 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 16.02.23 10:16, Muhammad Usama Anjum wrote: > mwriteprotect_range() errors out if [start, end) doesn't fall in one > VMA. We are facing a use case where multiple VMAs are present in one > range of interest. For example, the following pseudocode reproduces the > error which we are trying to fix: > - Allocate memory of size 16 pages with PROT_NONE with mmap > - Register userfaultfd > - Change protection of the first half (1 to 8 pages) of memory to > PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs. > - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors > out. I think, in QEMU, with partial madvise()/mmap(MAP_FIXED) while handling memory remapping during reboot to discard pages with memory errors, it would be possible that we get multiple VMAs and could not enable uffd-wp for background snapshots anymore. So this change makes sense to me. Especially, because userfaultfd_register() seems to already properly handle multi-VMA ranges correctly. It traverses the VMA list twice ... but also holds the mmap lock in write mode. > > This is a simple use case where user may or may not know if the memory > area has been divided into multiple VMAs. > > We need an implementation which doesn't disrupt the already present > users. So keeping things simple, stop going over all the VMAs if any one > of the VMA hasn't been registered in WP mode. While at it, remove the > un-needed error check as well. > > Reported-by: Paul Gofman > Signed-off-by: Muhammad Usama Anjum > --- Acked-by: David Hildenbrand -- Thanks, David / dhildenb