Message-ID: <CAGsJ_4y=5xvZsWWiZoGh6eqhh+NXCsztyeLVbraUrxR=ruhWzw@mail.gmail.com>
Date: Fri, 6 Dec 2024 18:09:43 +1300
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, 
	Jonathan Corbet <corbet@....net>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	David Hildenbrand <david@...hat.com>, Lance Yang <ioworker0@...il.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Gavin Shan <gshan@...hat.com>, 
	Pankaj Raghav <kernel@...kajraghav.com>, Daniel Gomez <da.gomez@...sung.com>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 0/4] Control folio sizes used for page cache memory

It's unusual that many emails sent days ago are resurfacing on LKML.
Please ignore them.
By the way, does anyone know what happened?

On Fri, Dec 6, 2024 at 5:12 AM Barry Song <baohua@...nel.org> wrote:
>
> On Thu, Aug 8, 2024 at 10:27 PM Ryan Roberts <ryan.roberts@....com> wrote:
> >
> > On 17/07/2024 08:12, Ryan Roberts wrote:
> > > Hi All,
> > >
> > > This series is an RFC that adds sysfs and kernel cmdline controls to configure
> > > the set of allowed large folio sizes that can be used when allocating
> > > file-memory for the page cache. As part of the control mechanism, it provides
> > > for a special-case "preferred folio size for executable mappings" marker.
> > >
> > > I'm trying to solve 2 separate problems with this series:
> > >
> > > 1. Reduce pressure in iTLB and improve performance on arm64: This is a modified
> > > approach for the change at [1]. Instead of hardcoding the preferred executable
> > > folio size into the arch, user space can now select it. This decouples the arch
> > > code and also makes the mechanism more generic; it can be bypassed (the default)
> > > or any folio size can be set. For my use case, 64K is preferred, but I've also
> > > heard from Willy of a use case where putting all text into 2M PMD-sized folios
> > > is preferred. This approach avoids the need for synchonous MADV_COLLAPSE (and
> > > therefore faulting in all text ahead of time) to achieve that.
> >
> > Just a polite bump on this; I'd really like to get something like this merged to
> > help reduce iTLB pressure. We had a discussion at the THP Cabal meeting a few
> > weeks back without solid conclusion. I haven't heard any concrete objections
> > yet, but also only a luke-warm reception. How can I move this forwards?
>
> Hi Ryan,
>
> These requirements seem to apply to anon, swap, pagecache, and shmem to
> some extent. While the swapin_enabled knob was rejected, the shmem_enabled
> option is already in place.
>
> I wonder if it's possible to use the existing 'enabled' setting across
> all cases, as
> from an architectural perspective with cont-pte, pagecache may not differ from
> anon. The demand for reducing page faults, LRU overhead, etc., also seems
> quite similar.
>
> I imagine that once Android's file systems support mTHP, we’ll uniformly enable
> 64KB for anon, swap, shmem, and page cache. It should then be sufficient to
> enable all of them using a single knob:
> '/sys/kernel/mm/transparent_hugepage/hugepages-xxkB/enabled'.
>
> Is there anything that makes pagecache and shmem significantly different
> from anon? In my Android case, they all seem the same. However, I assume
> there might be other use cases where differentiating them is necessary?
>
> >
> > Thanks,
> > Ryan
> >
> >
> > >
> > > 2. Reduce memory fragmentation in systems under high memory pressure (e.g.
> > > Android): The theory goes that if all folios are 64K, then failure to allocate a
> > > 64K folio should become unlikely. But if the page cache is allocating lots of
> > > different orders, with most allocations having an order below 64K (as is the
> > > case today) then ability to allocate 64K folios diminishes. By providing control
> > > over the allowed set of folio sizes, we can tune to avoid crucial 64K folio
> > > allocation failure. Additionally I've heard (second hand) of the need to disable
> > > large folios in the page cache entirely due to latency concerns in some
> > > settings. These controls allow all of this without kernel changes.
> > >
> > > The value of (1) is clear and the performance improvements are documented in
> > > patch 2. I don't yet have any data demonstrating the theory for (2) since I
> > > can't reproduce the setup that Barry had at [2]. But my view is that by adding
> > > these controls we will enable the community to explore further, in the same way
> > > that the anon mTHP controls helped harden the understanding for anonymous
> > > memory.
> > >
> > > ---
> > > This series depends on the "mTHP allocation stats for file-backed memory" series
> > > at [3], which itself applies on top of yesterday's mm-unstable (650b6752c8a3). All
> > > mm selftests have been run; no regressions were observed.
> > >
> > > [1] https://lore.kernel.org/linux-mm/20240215154059.2863126-1-ryan.roberts@arm.com/
> > > [2] https://www.youtube.com/watch?v=ht7eGWqwmNs&list=PLbzoR-pLrL6oj1rVTXLnV7cOuetvjKn9q&index=4
> > > [3] https://lore.kernel.org/linux-mm/20240716135907.4047689-1-ryan.roberts@arm.com/
> > >
> > > Thanks,
> > > Ryan
> > >
> > > Ryan Roberts (4):
> > >   mm: mTHP user controls to configure pagecache large folio sizes
> > >   mm: Introduce "always+exec" for mTHP file_enabled control
> > >   mm: Override mTHP "enabled" defaults at kernel cmdline
> > >   mm: Override mTHP "file_enabled" defaults at kernel cmdline
> > >
> > >  .../admin-guide/kernel-parameters.txt         |  16 ++
> > >  Documentation/admin-guide/mm/transhuge.rst    |  66 +++++++-
> > >  include/linux/huge_mm.h                       |  61 ++++---
> > >  mm/filemap.c                                  |  26 ++-
> > >  mm/huge_memory.c                              | 158 +++++++++++++++++-
> > >  mm/readahead.c                                |  43 ++++-
> > >  6 files changed, 329 insertions(+), 41 deletions(-)
> > >
> > > --
> > > 2.43.0
> > >
> >
>
> Thanks
> Barry
>

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