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(p200300d82f2849002c244e201f219fbd.dip0.t-ipconnect.de. [2003:d8:2f28:4900:2c24:4e20:1f21:9fbd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b5e8dc217esm14797863f8f.28.2025.07.15.07.06.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Jul 2025 07:06:31 -0700 (PDT) Message-ID: Date: Tue, 15 Jul 2025 16:06:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/5] add static PMD zero page support To: Lorenzo Stoakes , Pankaj Raghav Cc: Zi Yan , Suren Baghdasaryan , Ryan Roberts , Baolin Wang , Borislav Petkov , Ingo Molnar , "H . Peter Anvin" , Vlastimil Babka , Mike Rapoport , Dave Hansen , Michal Hocko , Andrew Morton , Thomas Gleixner , Nico Pache , Dev Jain , "Liam R . Howlett" , Jens Axboe , linux-kernel@vger.kernel.org, willy@infradead.org, linux-mm@kvack.org, x86@kernel.org, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Darrick J . Wong" , mcgrof@kernel.org, gost.dev@samsung.com, hch@lst.de, Pankaj Raghav References: <20250707142319.319642-1-kernel@pankajraghav.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: FQsYdcges88AK5QBAKWJPz6ALQp6ZolwdNkF2DViHsw_1752588394 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 0F30580004 X-Stat-Signature: u9aq6dxkgb6a31mgabtpu7zcbenjdwoj X-HE-Tag: 1752588399-810742 X-HE-Meta: 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 AzDjIKck qKpOe0+PpBc9UxBOEsvKzFvmE1jr2Bjh3JXYbmciEPm9bd+9wXKUIlolqK6KvkEQrSHE0sGyqYcxvfhg/f4vTorH+SSyteoRUho1hR450x1s5VDrho8e2hiCzb5IaED6xwqT8AUWWjlU3tM+lfo+D/5FiB6a+ovMtE0SY3LawY4ZjStiWtYIE9rP25fPezERufbg6EzpgXGMuT6MFL9ci6W+Cs7EO/Fq4FMMS5Ci29QKV5YHNk/WYZAciYGky63w7QPoA+KpNp1m4JYM5llgcOXsGSiBuR90dNMBMVosnH4Q3e1YRgoYia6byzus/DJtAu4eHQVm88seQa/sQzYlv0bHXepAGXEhyq7/ky6nsjlC7AQC+pmnlLhmn2/XidokrxfFxSybiNUfvkBUKPMfa0YdFxAe3+BixIRew2bOJc8rvbNTPa5lSGgzXIPw+nX1XypwGzYnEKMSJl4hTehL27f/2+g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 15.07.25 16:02, Lorenzo Stoakes wrote: > On Wed, Jul 09, 2025 at 10:03:51AM +0200, Pankaj Raghav wrote: >> Hi Zi, >> >>>> Add a config option STATIC_PMD_ZERO_PAGE that will always allocate the huge_zero_folio via >>>> memblock, and it will never be freed. >>> >>> Do the above users want a PMD sized zero page or a 2MB zero page? Because on systems with non >>> 4KB base page size, e.g., ARM64 with 64KB base page, PMD size is different. ARM64 with 64KB base >>> page has 512MB PMD sized pages. Having STATIC_PMD_ZERO_PAGE means losing half GB memory. I am >>> not sure if it is acceptable. >>> >> >> That is a good point. My intial RFC patches allocated 2M instead of a PMD sized >> page. >> >> But later David wanted to reuse the memory we allocate here with huge_zero_folio. So >> if this config is enabled, we simply just use the same pointer for huge_zero_folio. >> >> Since that happened, I decided to go with PMD sized page. >> >> This config is still opt in and I would expect the users with 64k page size systems to not enable >> this. >> >> But to make sure we don't enable this for those architecture, I could do a per-arch opt in with >> something like this[1] that I did in my previous patch: >> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig >> index 340e5468980e..c3a9d136ec0a 100644 >> --- a/arch/x86/Kconfig >> +++ b/arch/x86/Kconfig >> @@ -153,6 +153,7 @@ config X86 >> select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if X86_64 >> select ARCH_WANT_HUGETLB_VMEMMAP_PREINIT if X86_64 >> select ARCH_WANTS_THP_SWAP if X86_64 >> + select ARCH_HAS_STATIC_PMD_ZERO_PAGE if X86_64 >> select ARCH_HAS_PARANOID_L1D_FLUSH >> select ARCH_WANT_IRQS_OFF_ACTIVATE_MM >> select BUILDTIME_TABLE_SORT >> >> >> diff --git a/mm/Kconfig b/mm/Kconfig >> index 781be3240e21..fd1c51995029 100644 >> --- a/mm/Kconfig >> +++ b/mm/Kconfig >> @@ -826,6 +826,19 @@ config ARCH_WANTS_THP_SWAP >> config MM_ID >> def_bool n >> >> +config ARCH_HAS_STATIC_PMD_ZERO_PAGE >> + def_bool n > > Hm is this correct? arm64 supports mutliple page tables sizes, so while the > architecture might 'support' it, it will vary based on page size, so actually we > don't care about arch at all? > >> + >> +config STATIC_PMD_ZERO_PAGE >> + bool "Allocate a PMD page for zeroing" >> + depends on ARCH_HAS_STATIC_PMD_ZERO_PAGE > > Maybe need to just make this depend on !CONFIG_PAGE_SIZE_xx? I think at some point we discussed "when does the PMD-sized zeropage make *any* sense on these weird arch configs" (512MiB on arm64 64bit) No idea who wants to waste half a gig on that at runtime either. But yeah, we should let the arch code opt in whether it wants it or not (in particular, maybe only on arm64 with CONFIG_PAGE_SIZE_4K) -- Cheers, David / dhildenb