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

--
Thanks,

David / dhildenb

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