Re: [PATCH v4 0/5] batched remove rmap in try_to_unmap_one()

[David Hildenbrand <david@redhat.com>]

On 14.03.23 10:48, Matthew Wilcox wrote:
> On Tue, Mar 14, 2023 at 10:16:09AM +0100, David Hildenbrand wrote:
>> On 14.03.23 04:09, Yin Fengwei wrote:
>>>     For long term with wider adoption of large folio in kernel (like large
>>>     folio for anonymous page), MADV_PAGEOUT needs be updated to handle
>>>     large folio as whole to avoid splitting it always.
>>
>> Just curious what the last sentence implies. Large folios are supposed to be
>> a transparent optimization. So why should we pageout all surrounding
>> subpages simply because a single subpage was requested to be paged out? That
>> might harm performance of some workloads ... more than the actual split.
>>
>> So it's not immediately obvious to me why "avoid splitting" is the correct
>> answer to the problem at hand.
> 
> Even if your madvise() call says to pageout all pages covered by a
> folio, the current code will split it.  That's what needs to be fixed.

Agreed, if possible in the future (swap handling ...).

> 
> At least for anonymous pages, using large folios is an attempt to treat
> all pages in a particular range the same way.  If the user says to only
> page out some of them, that's a big clue that these pages are different
> from the other pages, and so we should split a folio where the madvise
> call does not cover every page in the folio.

Agreed.

-- 
Thanks,

David / dhildenb