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Message-ID: <aa326283-468f-6c40-4c47-de7cf7cc5994@redhat.com>
Date: Wed, 3 May 2023 16:31:36 +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 v8 1/3] mm/mmap: separate writenotify and dirty tracking
logic
On 03.05.23 00:51, Lorenzo Stoakes wrote:
> vma_wants_writenotify() is specifically intended for setting PTE page table
> flags, accounting for existing page table flag state and whether the
> filesystem performs dirty tracking.
>
> Separate out the notions of dirty tracking and 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() and vma_is_shared_writable()
> functions. This is necessary to avoid making vma_needs_dirty_tracking()
> needlessly complicated (e.g. passing flags or having it assume checks were
> already performed). This is small enough that it doesn't seem too
> egregious.
>
> We check to ensure the mapping is shared writable, as any GUP caller will
> be safe - MAP_PRIVATE mappings will be CoW'd and read-only file-backed
> shared mappings are not permitted access, even with FOLL_FORCE.
>
> 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 | 53 ++++++++++++++++++++++++++++++++++------------
> 2 files changed, 41 insertions(+), 13 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..fa7442e44cc2 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1475,6 +1475,42 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> }
> #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>
> +/* Do VMA operations imply write notify is required? */
Nit: comment is superfluous, this is already self-documenting code.
> +static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
> +{
> + return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
> +}
> +
> +/* Is this VMA shared and writable? */
Nit: dito
> +static bool vma_is_shared_writable(struct vm_area_struct *vma)
> +{
> + return (vma->vm_flags & (VM_WRITE | VM_SHARED)) ==
> + (VM_WRITE | VM_SHARED);
> +}
> +
> +/*
> + * Does this VMA require the underlying folios to have their dirty state
> + * tracked?
> + */
Nit: dito
> +bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
> +{
> + /* Only shared, writable VMAs require dirty tracking. */
> + if (!vma_is_shared_writable(vma))
> + return false;
> +
> + /* Does the filesystem need to be notified? */
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> + return true;
> +
> + /* Specialty mapping? */
> + if (vma->vm_flags & VM_PFNMAP)
> + return false;
> +
> + /* Can the mapping track the dirty pages? */
> + return vma->vm_file && vma->vm_file->f_mapping &&
> + mapping_can_writeback(vma->vm_file->f_mapping);
> +}
> +
> /*
> * Some shared mappings will want the pages marked read-only
> * to track write events. If so, we'll downgrade vm_page_prot
> @@ -1483,21 +1519,18 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
> */
> int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> {
> - vm_flags_t vm_flags = vma->vm_flags;
> - const struct vm_operations_struct *vm_ops = vma->vm_ops;
> -
> /* If it was private or non-writable, the write bit is already clear */
> - if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
> + if (!vma_is_shared_writable(vma))
> return 0;
>
> /* The backer wishes to know when pages are first written to? */
> - if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
> + if (vm_ops_needs_writenotify(vma->vm_ops))
> return 1;
>
> /* The open routine did something to the protections that pgprot_modify
> * won't preserve? */
> if (pgprot_val(vm_page_prot) !=
> - pgprot_val(vm_pgprot_modify(vm_page_prot, vm_flags)))
> + pgprot_val(vm_pgprot_modify(vm_page_prot, vma->vm_flags)))
> return 0;
>
> /*
> @@ -1511,13 +1544,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> if (userfaultfd_wp(vma))
> return 1;
>
> - /* Specialty mapping? */
> - if (vm_flags & VM_PFNMAP)
> - return 0;
> -
> - /* Can the mapping track the dirty pages? */
> - return vma->vm_file && vma->vm_file->f_mapping &&
> - mapping_can_writeback(vma->vm_file->f_mapping);
> + return vma_needs_dirty_tracking(vma);
> }
>
> /*
We now have duplicate vma_is_shared_writable() and
vm_ops_needs_writenotify() checks ...
Maybe move the VM_PFNMAP and "/* Can the mapping track the dirty pages?
*/" checks into a separate helper and call that from both,
vma_wants_writenotify() and vma_needs_dirty_tracking() ?
In any case
Acked-by: David Hildenbrand <david@...hat.com>
--
Thanks,
David / dhildenb
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