linux-kernel - Re: [PATCH v3 00/34] New page table range API

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Message-ID: <5e2f8747-cff8-08ac-7280-f8978cbf56a8@arm.com>
Date:   Thu, 9 Mar 2023 11:09:04 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-arch@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 00/34] New page table range API

On 28/02/2023 21:37, Matthew Wilcox (Oracle) wrote:
> This patchset changes the API used by the MM to set up page table entries.
> The four APIs are:
>     set_ptes(mm, addr, ptep, pte, nr)
>     update_mmu_cache_range(vma, addr, ptep, nr)
>     flush_dcache_folio(folio)
>     flush_icache_pages(vma, page, nr)
> 
> flush_dcache_folio() isn't technically new, but no architecture
> implemented it, so I've done that for you.  The old APIs remain around
> but are mostly implemented by calling the new interfaces.
> 
> The new APIs are based around setting up N page table entries at once.
> The N entries belong to the same PMD, the same folio and the same VMA,
> so ptep++ is a legitimate operation, and locking is taken care of for
> you.  Some architectures can do a better job of it than just a loop,
> but I have hesitated to make too deep a change to architectures I don't
> understand well.
> 
> One thing I have changed in every architecture is that PG_arch_1 is now a
> per-folio bit instead of a per-page bit.  This was something that would
> have to happen eventually, and it makes sense to do it now rather than
> iterate over every page involved in a cache flush and figure out if it
> needs to happen.
> 
> The point of all this is better performance, and Fengwei Yin has
> measured improvement on x86.  I suspect you'll see improvement on
> your architecture too.  Try the new will-it-scale test mentioned here:
> https://lore.kernel.org/linux-mm/20230206140639.538867-5-fengwei.yin@intel.com/
> You'll need to run it on an XFS filesystem and have
> CONFIG_TRANSPARENT_HUGEPAGE set.
> 
> For testing, I've only run the code on x86.  If an x86->foo compiler
> exists in Debian, I've built defconfig.  I'm relying on the buildbots
> to tell me what I missed, and people who actually have the hardware to
> tell me if it actually works.
> 
> I'd like to get this into the MM tree soon after the current merge window
> closes, so quick feedback would be appreciated.

I've boot-tested the series (with the Yin's typo fix for patch 32) on arm64 FVP
and Ampere Altra. On the Altra, I also ran page_fault4 from will-it-scale, and
see ~35% improvement from this series. So:

Tested-by: Ryan Roberts <ryan.roberts@....com>

Thanks,
Ryan