netdev - Re: [PATCH v7 1/3] mm/mmap: separate writenotify and dirty tracking logic

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Message-ID: <56696a72-24fa-958e-e6a1-7a17c9e54081@redhat.com>
Date:   Tue, 2 May 2023 18:38:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 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 ...

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.

Or was there a good reason to disallow private mappings as well?

-- 
Thanks,

David / dhildenb