Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

From: Matthew Wilcox <willy@infradead.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: "Yin, Fengwei" <fengwei.yin@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	David Hildenbrand <david@redhat.com>, Yu Zhao <yuzhao@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Yang Shi <shy828301@gmail.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance
Date: Wed, 5 Jul 2023 00:57:25 +0100	[thread overview]
Message-ID: <ZKSx5Udt3Oh8Kr35@casper.infradead.org> (raw)
In-Reply-To: <f364d9f1-8f7b-f531-ab9e-400c57a60c16@arm.com>

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.