Re: [RFC PATCH v1 0/4] Control folio sizes used for page cache memory

[Barry Song <21cnbao@gmail.com>]

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@kernel.org> wrote:
>
> On Thu, Aug 8, 2024 at 10:27 PM Ryan Roberts <ryan.roberts@arm.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
>