linux-kernel - Re: [PATCH v4 0/7] Split a folio to any lower order folios

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Date: Wed, 14 Feb 2024 11:55:15 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>, linux-mm@...ck.org,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Yu Zhao <yuzhao@...gle.com>,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Michal Koutný <mkoutny@...e.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Zach O'Keefe
 <zokeefe@...gle.com>, Hugh Dickins <hughd@...gle.com>,
 Mcgrof Chamberlain <mcgrof@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 0/7] Split a folio to any lower order folios

On 14.02.24 11:50, Ryan Roberts wrote:
> On 13/02/2024 22:31, Zi Yan wrote:
>> On 13 Feb 2024, at 17:21, David Hildenbrand wrote:
>>
>>> On 13.02.24 22:55, Zi Yan wrote:
>>>> From: Zi Yan <ziy@...dia.com>
>>>>
>>>> Hi all,
>>>>
>>>> File folio supports any order and multi-size THP is upstreamed[1], so both
>>>> file and anonymous folios can be >0 order. Currently, split_huge_page()
>>>> only splits a huge page to order-0 pages, but splitting to orders higher than
>>>> 0 is going to better utilize large folios. In addition, Large Block
>>>> Sizes in XFS support would benefit from it[2]. This patchset adds support for
>>>> splitting a large folio to any lower order folios and uses it during file
>>>> folio truncate operations.
>>>>
>>>> For Patch 6, Hugh did not like my approach to minimize the number of
>>>> folios for truncate[3]. I would like to get more feedback, especially
>>>> from FS people, on it to decide whether to keep it or not.
>>>
>>> I'm curious, would it make sense to exclude the "more" controversial parts (i.e., patch #6) for now, and focus on the XFS use case only?
>>
>> Sure. Patch 6 was there to make use of split_huge_page_to_list_to_order().
>> Now we have multi-size THP and XFS use cases, it can be dropped.
> 
> What are your plans for how to determine when to split THP and to what order? I
> don't see anything in this series that would split anon THP to non-zero order?
> 
> We have talked about using hints from user space in the past (e.g.  mremap,
> munmap, madvise, etc). But chrome has a use case where it temporarily mprotects
> a single (4K) page as part of garbage collection (IIRC). If you eagerly split on
> that hint, you will have lost the benefits of the large folio when it later
> mprotects back to the original setting.

Not only that, splitting will make some of these operations more 
expensive, possibly with no actual benefit.

> 
> I guess David will suggest this would be a good use case for the khugepaged-lite
> machanism we have been talking about. I dunno - it seems wasteful to split then
> collapse again.

I agree. mprotect() and even madvise(), ... might not be good candidates 
for splitting. mremap() likely is, if the folio is mapped exclusively. 
MADV_DONTNEED/munmap()/mlock() might be good candidates (again, if 
mapped exclusively). This will need a lot of thought I'm afraid (as you 
say, deferred splitting is another example).

-- 
Cheers,

David / dhildenb