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Message-ID: <bf04a98a-9de6-4532-a36c-59572d22dd7c@lucifer.local> Date: Tue, 2 May 2023 18:09:14 +0100 From: Lorenzo Stoakes <lstoakes@...il.com> To: David Hildenbrand <david@...hat.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, Oleg Nesterov <oleg@...hat.com>, Jason Gunthorpe <jgg@...dia.com>, John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>, "Kirill A . Shutemov" <kirill@...temov.name>, Pavel Begunkov <asml.silence@...il.com>, Mika Penttila <mpenttil@...hat.com>, Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>, Peter Xu <peterx@...hat.com>, Matthew Rosato <mjrosato@...ux.ibm.com>, "Paul E . McKenney" <paulmck@...nel.org>, Christian Borntraeger <borntraeger@...ux.ibm.com> Subject: Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic 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 <lstoakes@...il.com> > > > Reviewed-by: John Hubbard <jhubbard@...dia.com> > > > Reviewed-by: Mika Penttilä <mpenttil@...hat.com> > > > Reviewed-by: Jan Kara <jack@...e.cz> > > > Reviewed-by: Jason Gunthorpe <jgg@...dia.com> > > > --- > > > 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 > >
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