linux-kernel - Re: [PATCH v3 0/7] Split a folio to any lower order folios

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Date:   Sun, 16 Apr 2023 11:11:49 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Zi Yan <ziy@...dia.com>, Zi Yan <zi.yan@...t.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Yang Shi <shy828301@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        linux-mm@...ck.org,
        "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>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 0/7] Split a folio to any lower order folios

On Tue, 4 Apr 2023, Andrew Morton wrote:
> On Mon,  3 Apr 2023 16:18:32 -0400 Zi Yan <zi.yan@...t.com> wrote:
> 
> > File folio supports any order and people would like to support flexible orders
> > for anonymous folio[1] too. Currently, split_huge_page() only splits a huge
> > page to order-0 pages, but splitting to orders higher than 0 is also 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 queue
> it up for some testing now, but I do ask that some reviewers go through
> 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: done
because it could be done, it seems to me, but liable to add surprises.

The cover letter says "splitting to orders higher than 0 is also useful",
but it's not clear why; and the infrastructure provided seems unsuited
to the one use provided - I'll say more on that truncation patch.

Thanks,
Hugh