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From:   Zi Yan <ziy@nvidia.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Yang Shi <shy828301@gmail.com>, Yu Zhao <yuzhao@google.com>,
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        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Ryan Roberts <ryan.roberts@arm.com>,
        =?utf-8?q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
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        linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v3 0/7] Split a folio to any lower order folios
Date:   Tue, 18 Apr 2023 10:00:43 -0400
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On 18 Apr 2023, at 6:29, David Hildenbrand wrote:

> On 17.04.23 21:26, Zi Yan wrote:
>> On 17 Apr 2023, at 10:20, David Hildenbrand wrote:
>>
>>> On 16.04.23 20:11, Hugh Dickins wrote:
>>>> On Tue, 4 Apr 2023, Andrew Morton wrote:
>>>>> On Mon,  3 Apr 2023 16:18:32 -0400 Zi Yan <zi.yan@sent.com> wrote:
>>>>>
>>>>>> File folio supports any order and people would like to support fle=
xible orders
>>>>>> for anonymous folio[1] too. Currently, split_huge_page() only spli=
ts a huge
>>>>>> page to order-0 pages, but splitting to orders higher than 0 is al=
so useful.
>>>>>> This patchset adds support for splitting a huge page to any lower =
order pages
>>>>>> and uses it during file folio truncate operations.
>>>>>
>>>>> This series (and its v1 & v2) don't appear to have much in the way =
of
>>>>> detailed review.  As it's at v3 and has been fairly stable I'll que=
ue
>>>>> it up for some testing now, but I do ask that some reviewers go thr=
ough
>>>>> it please.
>>>>
>>>> Andrew, please don't let this series drift into 6.4-rc1.
>>>>
>>>> I've seen a bug or two (I'll point out in response to those patches)=
,
>>>> but overall I don't see what the justification for the series is: do=
ne
>>>> because it could be done, it seems to me, but liable to add surprise=
s.
>>>>
>>>> The cover letter says "splitting to orders higher than 0 is also use=
ful",
>>>> but it's not clear why; and the infrastructure provided seems unsuit=
ed
>>>> to the one use provided - I'll say more on that truncation patch.
>>>
>>> I agree. Maybe this patch set is something we want to have in the fut=
ure once actual consumers that can benefit are in place, such that we can=
 show actual performance numbers with/without.
>>
>> Ryan is working on large folio for anonymous pages and has shown promi=
sing performance
>> results[1]. This patchset would avoid getting base pages during split =
if possible.
>>
> Yes, I know. And it would be great to get some actual numbers with/with=
out your patches to show that this optimization actually matters in pract=
ice.

Sure. Will try to add perf numbers in the next version.

>
> Unrelated, your cover letter mentions "file folio truncate operations."=
=2E Would it also apply to FALLOC_FL_PUNCH_HOLE, when partially zapping a=
 THP?

Yes. In the self tests, I have
fallocate(fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE, offset[j], len[j]=
);
and it uses the truncate operation I updated in patch 6.

>
>>>
>>> Until then, "365 insertions(+), 68 deletions(-)" certainly needs some=
 reasonable motivation.
>>
>> Come on. 225 out of 365 insertions are self tests code. We need motiva=
tion to add
>> testing code?
>
> Well, if you add feature X and the tests target feature X, then certain=
ly having the tests require the same motivation as feature X. But yeah, t=
he actual kernel code change is smaller than it looks at first sight.
>
> -- =

> Thanks,
>
> David / dhildenb


--
Best Regards,
Yan, Zi

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