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Message-ID: <20230424134026.di6nf2an3a2g63a6@box.shutemov.name> Date: Mon, 24 Apr 2023 16:40:26 +0300 From: "Kirill A. Shutemov" <kirill@...temov.name> To: Lorenzo Stoakes <lstoakes@...il.com> Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Matthew Wilcox <willy@...radead.org>, Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>, Leon Romanovsky <leon@...nel.org>, Christian Benvenuti <benve@...co.com>, Nelson Escobar <neescoba@...co.com>, Bernard Metzler <bmt@...ich.ibm.com>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Bjorn Topel <bjorn@...nel.org>, Magnus Karlsson <magnus.karlsson@...el.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Jonathan Lemon <jonathan.lemon@...il.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Christian Brauner <brauner@...nel.org>, Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org Subject: Re: [PATCH] mm/gup: disallow GUP writing to file-backed mappings by default On Mon, Apr 24, 2023 at 01:31:56PM +0100, Lorenzo Stoakes wrote: > On Mon, Apr 24, 2023 at 03:02:47PM +0300, Kirill A. Shutemov wrote: > > On Sat, Apr 22, 2023 at 02:37:05PM +0100, Lorenzo Stoakes wrote: > > > @@ -959,16 +959,46 @@ static int faultin_page(struct vm_area_struct *vma, > > > return 0; > > > } > > > > > > +/* > > > + * Writing to file-backed mappings using GUP is a fundamentally broken operation > > > + * as kernel write access to GUP mappings may not adhere to the semantics > > > + * expected by a file system. > > > + * > > > + * In most instances we disallow this broken behaviour, however there are some > > > + * exceptions to this enforced here. > > > + */ > > > +static inline bool can_write_file_mapping(struct vm_area_struct *vma, > > > + unsigned long gup_flags) > > > +{ > > > + struct file *file = vma->vm_file; > > > + > > > + /* If we aren't pinning then no problematic write can occur. */ > > > + if (!(gup_flags & (FOLL_GET | FOLL_PIN))) > > > + return true; > > > + > > > + /* Special mappings should pose no problem. */ > > > + if (!file) > > > + return true; > > > + > > > + /* Has the caller explicitly indicated this case is acceptable? */ > > > + if (gup_flags & FOLL_ALLOW_BROKEN_FILE_MAPPING) > > > + return true; > > > + > > > + /* shmem and hugetlb mappings do not have problematic semantics. */ > > > + return vma_is_shmem(vma) || is_file_hugepages(file); > > > > Can this be generalized to any fs that doesn't have vm_ops->page_mkwrite()? > > > > Something more general would be preferable, however I believe there were > concerns broader than write notify, for instance not correctly marking the > folio dirty after writing to it, though arguably the caller should > certainly be ensuring that (and in many cases, do). It doesn't make much sense to me. Shared writable mapping without page_mkwrite (or pfn_write) will setup writeable PTE even on read faults[1], so you will not get the page dirty, unless you scan page table entries for dirty bit. [1] See logic around vm_page_prot vs. vma_wants_writenotify(). -- Kiryl Shutsemau / Kirill A. Shutemov
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