Re: [PATCH v1] mm/gup: adjust stale comment for RCU GUP-fast

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 9/5/22 2:08 PM, David Hildenbrand wrote:
> On 04.09.22 18:52, Aneesh Kumar K V wrote:
>> On 9/2/22 12:02 PM, David Hildenbrand wrote:
>>> On 01.09.22 20:35, Yang Shi wrote:
>>>> On Thu, Sep 1, 2022 at 11:07 AM Peter Xu <peterx@redhat.com> wrote:
>>>>>
>>>>> On Thu, Sep 01, 2022 at 10:50:48AM -0700, Yang Shi wrote:
>>>>>> Yeah, because THP collapse does copy the data before clearing pte. If
>>>>>> we want to remove pmdp_collapse_flush() by just clearing pmd, we
>>>>>> should clear *AND* flush pte before copying the data IIRC.
>>>>>
>>>>> Yes tlb flush is still needed.  IIUC the generic pmdp_collapse_flush() will
>>>>> still be working (with the pte level flushing there) but it should just
>>>>> start to work for all archs, so potentially we could drop the arch-specific
>>>>> pmdp_collapse_flush()s, mostly the ppc impl.
>>>>
>>>> I'm don't know why powperpc needs to have its specific
>>>> pmdp_collapse_flush() in the first place, not only the mandatory IPI
>>>> broadcast, but also the specific implementation of pmd tlb flush. But
>>>> anyway the IPI broadcast could be removed at least IMO.
>>>>
>>>
>>> pmdp_collapse_flush() is overwritten on book3s only. It either translates
>>> to radix__pmdp_collapse_flush() or hash__pmdp_collapse_flush().
>>>
>>>
>>> radix__pmdp_collapse_flush() has a comment explaining the situation:
>>>
>>>
>>> +       /*
>>> +        * pmdp collapse_flush need to ensure that there are no parallel gup
>>> +        * walk after this call. This is needed so that we can have stable
>>> +        * page ref count when collapsing a page. We don't allow a collapse page
>>> +        * if we have gup taken on the page. We can ensure that by sending IPI
>>> +        * because gup walk happens with IRQ disabled.
>>> +        */
>>>
>>>
>>> The comment for hash__pmdp_collapse_flush() is a bit more involved:
>>>
>>>     /*
>>>      * Wait for all pending hash_page to finish. This is needed
>>>      * in case of subpage collapse. When we collapse normal pages
>>>      * to hugepage, we first clear the pmd, then invalidate all
>>>      * the PTE entries. The assumption here is that any low level
>>>      * page fault will see a none pmd and take the slow path that
>>>      * will wait on mmap_lock. But we could very well be in a
>>>      * hash_page with local ptep pointer value. Such a hash page
>>>      * can result in adding new HPTE entries for normal subpages.
>>>      * That means we could be modifying the page content as we
>>>      * copy them to a huge page. So wait for parallel hash_page
>>>      * to finish before invalidating HPTE entries. We can do this
>>>      * by sending an IPI to all the cpus and executing a dummy
>>>      * function there.
>>>      */
>>>
>>> I'm not sure if that implies that the IPI is needed for some other hash-magic.
>>>
>>> Maybe Aneesh can clarify.
>>>
>>
>> We still need the IPI for the hash. Another reason for architecture to override that
>> function is to help them use the right page size when flushing the TLB.
> 
> Thanks for clarifying. So the radix variant wouldn't need the IPI anymore, once GUP-fast is handled differently, correct?
> 

yes. With this patch https://lkml.kernel.org/r/20220901222707.477402-1-shy828301@gmail.com we can remove the
serialize_against_pte_lookup(vma->vm_mm); in radix__pmdp_collapse_flush()

-aneesh