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Shutemov" , Catalin Marinas Subject: Re: [PATCH] mm: Fix __wp_page_copy_user fallback path for remote mm Message-Id: <20241104194242.2456fbd25cd027b338c07855@linux-foundation.org> In-Reply-To: <251d19c0-f788-4291-a4b3-d6f8a9b3b4f1@asahilina.net> References: <20241101-mm-remote-pfn-v1-1-080b609270b7@asahilina.net> <20241101120717.11db30a5abc6378da7910719@linux-foundation.org> <251d19c0-f788-4291-a4b3-d6f8a9b3b4f1@asahilina.net> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: E0793C0008 X-Stat-Signature: th759u3gpkugj1f183opej3agtf9wcgw X-Rspam-User: X-HE-Tag: 1730778131-96466 X-HE-Meta: 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 ltEUCvxS XvyonWd1huMixYfj1riMSCL17rKNzeEzz38VyvoMXYa7lZkNgNWFIwxdwXovMESXArUX64pL4+N9QyX9GnpSmNe+/FRTXoiYenAi3JHN4yI/vr0WVRzG2mPgr0MGXtBVJu1uE3QsozjkkJGiZeLPVdKGDhQqkz6lA9QAk0jxOs/gUuqG/vkDlDeEyzspnYBZ5YZyuCsR+1kVb+bLdpKIoUSIv4fA0mAeCfQECVHHrMGo9Fw9pI2raPMUNcFtFX9zhb//bE1XPjTyROyeSUsu8B1yoeSS6K7W6tUvDpxmlzWps73ijPCmIyTinR+qoK2opS075wcQRUjQW1SFgGa/azr3QY5hl0nbBTBJ8 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: List-Subscribe: List-Unsubscribe: On Sat, 2 Nov 2024 06:18:54 +0900 Asahi Lina wrote: > > > On 11/2/24 4:07 AM, Andrew Morton wrote: > > On Fri, 01 Nov 2024 21:08:02 +0900 Asahi Lina wrote: > > > >> If the source page is a PFN mapping, we copy back from userspace. > >> However, if this fault is a remote access, we cannot use > >> __copy_from_user_inatomic. Instead, use access_remote_vm() in this case. > >> > >> Fixes WARN and incorrect zero-filling when writing to CoW mappings in > >> a remote process, such as when using gdb on a binary present on a DAX > >> filesystem. > >> > >> [ 143.683782] ------------[ cut here ]------------ > >> [ 143.683784] WARNING: CPU: 1 PID: 350 at mm/memory.c:2904 __wp_page_copy_user+0x120/0x2bc > >> > >> ... > >> > > > > Thanks. I assume we should backport this into earlier kernels? > > > > If so, a Fixes: target is desired, to tell people how far back in time > > it should be ported. > > I think so? I'm not sure how back the bug goes though, possibly a long > time... > > > I think it's > > > > 83d116c53058 ("mm: fix double page fault on arm64 if PTE_AF is cleared"). > > That doesn't sound right. The old code prior to the patch still had the > __copy_from_user_inatomic() fallback path so it should still have the > same problem. That fallback goes back to: > > 6aab341e0a28 ("mm: re-architect the VM_UNPAGED logic") > > But the ptrace code back then doesn't seem to be using that codepath at > all, so that's meaningless. I think this is the proper tag: > > 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings") > > That's when GUP started working for DAX mappings at all, and if my > reading of the code is correct, at that point do_wp_page() was only > grabbing the struct page for normal pages to pass to wp_page_copy() > (triggering the fallback path for DAX mappings). The code has moved > around a lot today but has the same logic, so I think it's been broken > since then. Cool, thanks. > Should I resend it with the Fixes tag? That's OK, I edited the mm.git changelog.