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

From: "Huang, Ying" <ying.huang@intel.com>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	 Matthew Wilcox <willy@infradead.org>,
	 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	 Yin Fengwei <fengwei.yin@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: Fri, 07 Jul 2023 16:01:45 +0800	[thread overview]
Message-ID: <87edlkgnfa.fsf@yhuang6-desk2.ccr.corp.intel.com> (raw)
In-Reply-To: <20230703135330.1865927-5-ryan.roberts@arm.com> (Ryan Roberts's message of "Mon, 3 Jul 2023 14:53:29 +0100")

Ryan Roberts <ryan.roberts@arm.com> writes:

> Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
> allocated in large folios of a specified order. All pages of the large
> folio are pte-mapped during the same page fault, significantly reducing
> the number of page faults. The number of per-page operations (e.g. ref
> counting, rmap management lru list management) are also significantly
> reduced since those ops now become per-folio.

I likes the idea to share as much code as possible between large
(anonymous) folio and THP.  Finally, THP becomes just a special kind of
large folio.

Although we can use smaller page order for FLEXIBLE_THP, it's hard to
avoid internal fragmentation completely.  So, I think that finally we
will need to provide a mechanism for the users to opt out, e.g.,
something like "always madvise never" via
/sys/kernel/mm/transparent_hugepage/enabled.  I'm not sure whether it's
a good idea to reuse the existing interface of THP.

Best Regards,
Huang, Ying