linux-kernel - Re: [PATCH v3 6/7] mm: truncate: split huge page cache page to a non-zero order if possible.

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Date:   Mon, 17 Apr 2023 18:05:51 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Zi Yan <ziy@...dia.com>
cc:     Hugh Dickins <hughd@...gle.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 6/7] mm: truncate: split huge page cache page to a
 non-zero order if possible.

On Mon, 17 Apr 2023, Zi Yan wrote:
> 
> What do you think? If you have a better idea, I am all ears. And if you
> are willing to help me review the more complicated code changes, I am
> more than happy to implement it in the next version. :)

Sorry, no, not me.  You'll have to persuade someone else that "optimizing"
truncation is a worthwhile path to pursue (and to pursue now) - I was just
trying to illustrate that the current patchset didn't seem very useful.

But don't throw your work away.  I expect some of it can become useful
later e.g. once most of the main filesystems support large folios, and
the complication of CONFIG_READ_ONLY_THP_FOR_FS can be deleted.

I doubt my "minimizing the number of folios" approach would be worth
the effort; I think more likely that we shall settle on an intermediate
folio size (between 4K and THP: maybe 64K, but not necessarily the same
on all machines or all workloads) to aim for, and then maybe truncation
of THP would split to those units.  But it's not a job I shall get into
- I'll just continue to report and try to fix what bugs I see.

Hugh