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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id s18-20020ac87592000000b003b868cdc689sm1889205qtq.5.2023.02.16.12.25.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Feb 2023 12:25:55 -0800 (PST) Date: Thu, 16 Feb 2023 15:25:54 -0500 From: Peter Xu To: David Hildenbrand Cc: Muhammad Usama Anjum , Andrew Morton , kernel@collabora.com, Paul Gofman , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 1/2] mm/userfaultfd: Support WP on multiple VMAs Message-ID: References: <20230216091656.2045471-1-usama.anjum@collabora.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: dio7wmo511dhf7n8ip9ys9x9bs8ue41m X-Rspamd-Queue-Id: 74507C0015 X-HE-Tag: 1676579161-509938 X-HE-Meta: U2FsdGVkX1/+DX6l6tf2zU4vQNfeCyVxmV/wGMOlrzBnQmEy6NNyOm5IhTawDWie9FzmWmsU466jvPEaQ0tJa5lDpg/UXhLtxgUBHb5+ME/JQOofWPS4cjIrlHyuu7n7umUev3iwlWci4G1eVQyovOAzWWQnobsGJw94G1UHrQ5N1UUpPogtYzlSplRNiT8g4b2zDpDulRcKwJZFEV+03RlCD8FlIjZrk2G609Z594BM/A92V02neQcE0F9nzjenzWOjTCzGMU3ro211rE6QAIEeXIYpn4vrPU5wykkwKBNV17QHZNKC4VvoMxax1fC9gLqnpsHZwf3uDhBcN81e703gAWs/H88IYGOux+MBmGfewLKkK0InunDhsZhN9kYqa8XnFlD7cHxCbtgSbCic5i35u1d3nxG6AzLM4ZlNjzLBmjFxR7OaouxXkFpFYBrRSkXAduSwU1n5GkJrkGmMvgmAzw/7z+glNOfQEbXGZzoQHUw5q0+1YeSlnhDabce0F1sQ/2yx1oL6Zkf9FJWEKEn9Cohu6leYB12ZNKro31boZLyghRKmUBZWV1QzWblhnh1+I9/NMFrDXDi4xm2Ed7mvoNG/MsO8zRnKMwDsZGI0uK6b64iy+XcvHI/9XhjIUNuDjKLKRfhPlFVLYKirgM6gasOFGHlBC5j56QCP0M2jKv5UtagRnRTsW7iQiDX/KJ+l5gJsOUiqW6OXDW5SK6kqqKEKsTJh1DRoxmvbdoS6oAm+Zw7wc5wt2doFdKzTAVRv4fCI+I4bRhUoTxxztVeNNKYyegZI+95osmKkxTrumTI1d5e8XUr3+Cttrw7CrjmkwNaqwDlOvD0HH1fmDk4YPPg4ZGR5qZ3iAo4qBXwVLcFqHHJ1hGlbfi/xnJu3CVz5D6ha74fR9QpvD5AGB8nIeF26SUZIzPaUC72uVO6WPsI4zKSV9xz7+hAp8pdlOS4DDFueOwSn00gfeAm lArQ3ijN 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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 Thu, Feb 16, 2023 at 10:37:36AM +0100, David Hildenbrand wrote: > 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. Any pointer for this one? > > 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 Acked-by: Peter Xu Thanks, -- Peter Xu