Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 17 Apr 2023 18:05:51 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Zi Yan <ziy@nvidia.com>
cc:     Hugh Dickins <hughd@google.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Yang Shi <shy828301@gmail.com>, Yu Zhao <yuzhao@google.com>,
        linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Ryan Roberts <ryan.roberts@arm.com>,
        =?ISO-8859-15?Q?Michal_Koutn=FD?= <mkoutny@suse.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Zach O'Keefe <zokeefe@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v3 6/7] mm: truncate: split huge page cache page to a
 non-zero order if possible.
In-Reply-To: <0AC0520E-1BD2-497E-A7ED-05394400BFC9@nvidia.com>
Message-ID: <8dbb3c83-48d-93d-3e3-4c833b81904@google.com>
References: <20230403201839.4097845-1-zi.yan@sent.com> <20230403201839.4097845-7-zi.yan@sent.com> <9dd96da-efa2-5123-20d4-4992136ef3ad@google.com> <0AC0520E-1BD2-497E-A7ED-05394400BFC9@nvidia.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

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