linux-kernel - Re: [PATCH v2 4/5] mm: FLEXIBLE

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Date:   Wed, 5 Jul 2023 00:57:25 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     "Yin, Fengwei" <fengwei.yin@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

On Tue, Jul 04, 2023 at 03:20:35PM +0100, Ryan Roberts wrote:
> On 04/07/2023 04:45, Yin, Fengwei wrote:
> > 
> > On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> >> Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
> > THP is for huge page which is 2M size. We are not huge page here. But
> > I don't have good name either.
> 
> Is that really true? On arm64 with 16K pages, huge pages are 32M and with 64K
> base page, they are 512M. So huge pages already have a variable size. And they
> sometimes get PTE-mapped. So can't we just think of this as an extension of the
> THP feature?

The confusing thing is that we have counters for the number of THP
allocated (and number of THP mapped), and for those we always use
PMD-size folios.

If we must have a config option, then this is ANON_LARGE_FOLIOS.

But why do we need a config option?  We don't have one for the
page cache, and we're better off for it.  Yes, it depends on
CONFIG_TRANSPARENT_HUGEPAGE today, but that's more of an accidental
heritage, and it'd be great to do away with that dependency eventually.

Hardware support isn't needed.  Large folios benefit us from a software
point of view.  if we need a chicken bit, we can edit the source code
to not create anon folios larger than order 0.