Re: [PATCH v4 3/3] mm: Batch-zap large anonymous folio PTE mappings

[Ryan Roberts <ryan.roberts@arm.com>]

On 27/07/2023 18:22, Yu Zhao wrote:
> On Thu, Jul 27, 2023 at 8:18 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> This allows batching the rmap removal with folio_remove_rmap_range(),
>> which means we avoid spuriously adding a partially unmapped folio to the
>> deferred split queue in the common case, which reduces split queue lock
>> contention.
>>
>> Previously each page was removed from the rmap individually with
>> page_remove_rmap(). If the first page belonged to a large folio, this
>> would cause page_remove_rmap() to conclude that the folio was now
>> partially mapped and add the folio to the deferred split queue. But
>> subsequent calls would cause the folio to become fully unmapped, meaning
>> there is no value to adding it to the split queue.
>>
>> A complicating factor is that for platforms where MMU_GATHER_NO_GATHER
>> is enabled (e.g. s390), __tlb_remove_page() drops a reference to the
>> page. This means that the folio reference count could drop to zero while
>> still in use (i.e. before folio_remove_rmap_range() is called). This
>> does not happen on other platforms because the actual page freeing is
>> deferred.
>>
>> Solve this by appropriately getting/putting the folio to guarrantee it
>> does not get freed early. Given the need to get/put the folio in the
>> batch path, we stick to the non-batched path if the folio is not large.
>> While the batched path is functionally correct for a folio with 1 page,
>> it is unlikely to be as efficient as the existing non-batched path in
>> this case.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> 
> This ad hoc patch looks unacceptable to me: we can't afford to keep
> adding special cases.
> 
> I vote for completely converting zap_pte_range() to use
> folio_remove_rmap_range(), and that includes tlb_flush_rmap_batch()
> and other types of large folios, not just anon. 

The intent of the change is to avoid the deferred split queue lock contention,
and this is only a problem for anon folios; page cache folios are never split in
this way. My intention was to do the smallest change to solve the problem. I
don't see the value in reworking a much bigger piece of the code, making it more
complex, when its not going to give any clear perf benefits.


Otherwise I'll leave
> it to Matthew and David.

If there is concensus that this is _required_ in order to merge this series,
then I guess I'll bite the bullet and do it. But my preference is to leave it
for if/when a reason is found that it is actually bringing benefit.