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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id g2-20020a5d5402000000b002da75c5e143sm31378906wrv.29.2023.05.02.10.09.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 May 2023 10:09:15 -0700 (PDT) Date: Tue, 2 May 2023 18:09:14 +0100 From: Lorenzo Stoakes To: David Hildenbrand Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , Jason Gunthorpe , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , Dave Chinner , Theodore Ts'o , Peter Xu , Matthew Rosato , "Paul E . McKenney" , Christian Borntraeger Subject: Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic Message-ID: References: <72a90af5a9e4445a33ae44efa710f112c2694cb1.1683044162.git.lstoakes@gmail.com> <56696a72-24fa-958e-e6a1-7a17c9e54081@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, May 02, 2023 at 05:53:46PM +0100, Lorenzo Stoakes wrote: > On Tue, May 02, 2023 at 06:38:53PM +0200, David Hildenbrand wrote: > > On 02.05.23 18:34, Lorenzo Stoakes wrote: > > > vma_wants_writenotify() is specifically intended for setting PTE page table > > > flags, accounting for existing PTE flag state and whether that might > > > already be read-only while mixing this check with a check whether the > > > filesystem performs dirty tracking. > > > > > > Separate out the notions of dirty tracking and a PTE write notify checking > > > in order that we can invoke the dirty tracking check from elsewhere. > > > > > > Note that this change introduces a very small duplicate check of the > > > separated out vm_ops_needs_writenotify(). This is necessary to avoid making > > > vma_needs_dirty_tracking() needlessly complicated (e.g. passing a > > > check_writenotify flag or having it assume this check was already > > > performed). This is such a small check that it doesn't seem too egregious > > > to do this. > > > > > > Signed-off-by: Lorenzo Stoakes > > > Reviewed-by: John Hubbard > > > Reviewed-by: Mika Penttilä > > > Reviewed-by: Jan Kara > > > Reviewed-by: Jason Gunthorpe > > > --- > > > include/linux/mm.h | 1 + > > > mm/mmap.c | 36 +++++++++++++++++++++++++++--------- > > > 2 files changed, 28 insertions(+), 9 deletions(-) > > > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > > index 27ce77080c79..7b1d4e7393ef 100644 > > > --- a/include/linux/mm.h > > > +++ b/include/linux/mm.h > > > @@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma, > > > #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \ > > > MM_CP_UFFD_WP_RESOLVE) > > > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma); > > > int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot); > > > static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma) > > > { > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 5522130ae606..295c5f2e9bd9 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg) > > > } > > > #endif /* __ARCH_WANT_SYS_OLD_MMAP */ > > > +/* Do VMA operations imply write notify is required? */ > > > +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops) > > > +{ > > > + return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite); > > > +} > > > + > > > +/* > > > + * Does this VMA require the underlying folios to have their dirty state > > > + * tracked? > > > + */ > > > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma) > > > +{ > > > > Sorry for not noticing this earlier, but ... > > pints_owed++ > > > > > what about MAP_PRIVATE mappings? When we write, we populate an anon page, > > which will work as expected ... because we don't have to notify the fs? > > > > I think you really also want the "If it was private or non-writable, the > > write bit is already clear */" part as well and remove "false" in that case. > > > > Not sure a 'write bit is already clear' case is relevant to checking > whether a filesystem dirty tracks? That seems specific entirely to the page > table bits. > > That's why I didn't include it, > > A !VM_WRITE shouldn't be GUP-writable except for FOLL_FORCE, and that > surely could be problematic if VM_MAYWRITE later? > > Thinking about it though a !VM_SHARE should probably can be safely assumed > to not be dirty-trackable, so we probably do need to add a check for > !VM_SHARED -> !vma_needs_dirty_tracking > On second thoughts, we explicitly check FOLL_FORCE && !is_cow_mapping() in check_vma_flags() so that case cannot occur. So actually yes we should probably include this on the basis of that and the fact that a FOLL_WRITE operation will CoW the MAP_PRIVATE mapping. This was an (over)abundance of caution. Will fix on respin. > > Or was there a good reason to disallow private mappings as well? > > > > Until the page is CoW'd walking the page tables will get you to the page > cache page right? This was the reason I (perhaps rather too quickly) felt > MAP_PRIVATE should be excluded. > > However a FOLL_WRITE would trigger CoW... and then we'd be trivially OK. > > So yeah, ok perhaps I dismissed that a little too soon. I was concerned > about some sort of egregious FOLL_FORCE case where somehow we'd end up with > the page cache folio. But actually, that probably can't happen... > > > -- > > Thanks, > > > > David / dhildenb > >