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,
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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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