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

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Date: Fri, 16 Feb 2024 11:06:20 +0100
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	David Hildenbrand <david@...hat.com>, Yang Shi <shy828301@...il.com>, Yu Zhao <yuzhao@...gle.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Ryan Roberts <ryan.roberts@....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, gost.dev@...sung.com
Subject: Re: [PATCH v4 0/7] Split a folio to any lower order folios

Hi Zi Yan,

On Tue, Feb 13, 2024 at 04:55:13PM -0500, 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.

I added your patches on top of my patches, but removed patch 6 and I
added this instead:

diff --git a/mm/truncate.c b/mm/truncate.c
index 725b150e47ac..dd07e2e327a8 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -239,7 +239,8 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
                folio_invalidate(folio, offset, length);
        if (!folio_test_large(folio))
                return true;
-       if (split_folio(folio) == 0)
+       if (split_folio_to_order(folio,
+                                mapping_min_folio_order(folio->mapping)) == 0)
                return true;
        if (folio_test_dirty(folio))
                return false;

I ran genric/476 fstest[1] with SOAK_DURATION set to 360 seconds. This
test uses fstress to do a lot of writes, truncate operations, etc. I ran
this on XFS with **64k block size on a 4k page size system**.

I recorded the vm event for split page and this was the result I got:

Before your patches:
root@...ian:~/xfstests# cat /proc/vmstat  | grep split
thp_split_page 0
thp_split_page_failed 5819

After your patches:
root@...ian:~/xfstests# cat /proc/vmstat  | grep split
thp_split_page 5846
thp_split_page_failed 20

Your patch series definitely helps with splitting the folios while still
maintaining the min_folio_order that LBS requires.

We are still discussing how to quantify this benefit in terms of some
metric with this support. If you have some ideas here, let me know.

I will run the whole xfstests tonight to check for any regressions.

--
Pankaj

[1] https://github.com/kdave/xfstests/blob/master/tests/generic/476