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Message-ID: <03e591ce-debc-bba1-c55e-ce590cc1f38d@redhat.com> Date: Tue, 2 May 2023 19:16:36 +0200 From: David Hildenbrand <david@...hat.com> 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, 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 02.05.23 19:09, Lorenzo Stoakes wrote: > 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++ Having tired eyes and jumping back and forth between tasks really seems to start getting expensive ;) >> >>> >>> 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. > Yes, we only allow to FOLL_FORCE write to (exclusive) anonymous pages that are mapped read-only. If it's not that, we trigger a (fake) write fault. -- Thanks, David / dhildenb
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