Re: [PATCH v2] mm: implement and use vma_anon_tracked(), silence KCSAN

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/14/26 17:21, Lorenzo Stoakes (ARM) wrote:
> On Thu, Sep 10, 2026 at 02:50:25PM +0100, Kiryl Shutsemau wrote:
>> On Thu, Sep 10, 2026 at 02:18:35PM +0100, Lorenzo Stoakes (ARM) wrote:
>>> Provide a function to abstract the common task of checking whether
>>> a VMA is faulted in or not.
>>>
>>> A VMA or mmap lock must be held when calling this function. For an attached
>>> VMA the transitions between unfaulted/faulted state are:
>>>
>>>        Transition        |              VMA/mmap Lock state
>>>  ------------------------|-----------------------------------------------
>>>    unfaulted to faulted  | write lock OR read lock + mm->page_table_lock
>>>    faulted to unfaulted  | write lock
>>>
>>> So vma_anon_tracked() never provides a false positive (the lock precludes
>>> it), but if only a read lock is held, a negative result must be re-checked
>>> with mm->page_table_lock held.
>>>
>>> Detached VMAs cannot be concurrently manipulated as they are removed from
>>> the maple tree so require no guarantees.
>>>
>>> Use data_race() to silence KCSAN about non-existent data races between
>>> concurrent vma->anon_vma read/write on optimistic fault tests.
>>>
>>> Also while here, const-ify vma_is_attached(), vma_assert_stabilised() and
>>> dependants.
>>
>> If you end up respinning anyway, this would be a natural separate
>> prep patch.
> 
> Yup will do, Pedro also asked for this!
> 
>>
>>> Finally, update the core VMA merge/split, rmap, mremap, KSM and fault
>>> preparation callers which test vma->anon_vma directly to use
>>> vma_anon_tracked() instead.
>>>
>>> Note that the lockless read in reusable_anon_vma() is doing more than
>>> checking whether the VMA is faulted - it is returning the anon_vma to be
>>> used on fault, so this check is not altered.
>>>
>>> There is one odd one out - file_backed_vma_is_retractable() - which holds
>>> neither a VMA nor mmap lock and is stabilised by the file rmap lock only.
>>>
>>> Therefore just add a comment to explain why the direct vma->anon_vma check
>>> is required.
>>>
>>> Reported-by: Guilherme Giacomo Simoes <trintaeoitogc@gmail.com>
>>> Closes: https://lore.kernel.org/all/20260829100034.423064-1-trintaeoitogc@gmail.com/
>>> Closes: https://lore.kernel.org/all/20260909115723.528501-1-trintaeoitogc@gmail.com/
>>> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>>> ---
>>> v2:
>>> - Renamed vma_is_faulted() to vma_anon_tracked() as per David.
>>
>> I don't particularly like vma_anon_tracked(), but naming is hard...
> 
> Yeah it was a compromise because David and I disagreed on naming (again ;)
> 
> I think vma_has_anon_rmap() is a better compromise?

Yeah let's go for that.

-- 
Cheers,

David