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b=S6cCxd+SxxusCjiRuyArac5qlK/iNc5LrpHJJ7i3t0muizcOZy7bg4MnhRpPUvFCx VzzXoI2Nyuv9WEHzb0tR5VeAaHfnyEgGY4VQI9jUpBch9cEOk8qgBbExafU+EuclzQ GLn0EYb12XUZAp7s5E+lmJhc9CsFwZXc2oVaxXJEaWuznYP7KZ6/OenrO1owA/X0B1 eh1M7fK3+iitM41MQMuXq9KmhYS7Q2mYwKyKwd/r4VCUKYKBSEuE3vT3t4Z9gKOdxD BGz/xV/6hjQewzIQ8MWBty1RwjOwQUcx7vyDRwLju4vMD1CMdiae+WpigXbjTEAs8/ xI3mqGLcJkdEw== Message-ID: <3453825f-2bc0-4d63-8731-3eaf9fc716a4@kernel.org> Date: Wed, 13 May 2026 19:00:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 20/22] mm/page_alloc: implement __GFP_UNMAPPED|__GFP_ZERO allocations Content-Language: en-US To: Brendan Jackman , Borislav Petkov , Dave Hansen , Peter Zijlstra , Andrew Morton , David Hildenbrand , Wei Xu , Johannes Weiner , Zi Yan , Lorenzo Stoakes Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, rppt@kernel.org, Sumit Garg , derkling@google.com, reijiw@google.com, Will Deacon , rientjes@google.com, patrick.roy@linux.dev, "Itazuri, Takahiro" , Andy Lutomirski , David Kaplan , Thomas Gleixner , Yosry Ahmed References: <20260320-page_alloc-unmapped-v2-0-28bf1bd54f41@google.com> <20260320-page_alloc-unmapped-v2-20-28bf1bd54f41@google.com> From: "Vlastimil Babka (SUSE)" In-Reply-To: <20260320-page_alloc-unmapped-v2-20-28bf1bd54f41@google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: AB643C001C X-Stat-Signature: g83iqf46pu7oe4s4xwbkygdqbnhx6cxc X-HE-Tag: 1778691657-444084 X-HE-Meta: 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 E2gaRjfd sy2DTVfIZx/oIpP+VPPzlkI8fdKnw0rhmi6PSOqqj6IPKT2oFdicVhDbdzdjd0DjXVmO9jNqSlekzjvVp81VZckbh0RxSZI/PTyMQgrPzIN4geQLMyVBSbFyYvsAMHRVF7MeK870UV+wlCyxyZz9yW5NEegKkZLiGub6MR+HSiwOlwYjkYxOLFcwvgnzcJfkoHVq94W13R+I5QRtmVe0/nulUpJ/kR7+O3Z4ypMGgmiu/JKaG1ou7rvm73mQNIBe4S4Do/TDTDxKCUpVq1TWfIgOuLifwPt0i6D/SPv3mVy4w50IJaodAJbzjKbJDP/m+iTHBiiHfhj3IxipK/jpZbZXax4lEPM2Hcjdy Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/20/26 19:23, Brendan Jackman wrote: > The pages being zeroed here are unmapped, so they can't be zeroed via > the direct map. Temporarily mapping them in the direct map is not > possible because: > > - In general this requires allocating pagetables, > > - Unmapping them would require a TLB shootdown, which can't be done in > general from the allocator (x86 requires IRQs on). > > Therefore, use the new mermap mechanism to zero these pages. > > The main mermap API is expected to fail very often. In order to avoid > needing to fail allocations when that happens, instead fallback to the > special mermap_get_reserved() variant, which is less efficient. > > Signed-off-by: Brendan Jackman > --- > arch/x86/include/asm/pgtable_types.h | 2 + > mm/Kconfig | 11 +++++- > mm/page_alloc.c | 76 +++++++++++++++++++++++++++++++----- > 3 files changed, 78 insertions(+), 11 deletions(-) > > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h > index 2ec250ba467e2..c3d73bdfff1fa 100644 > --- a/arch/x86/include/asm/pgtable_types.h > +++ b/arch/x86/include/asm/pgtable_types.h > @@ -223,6 +223,7 @@ enum page_cache_mode { > #define __PAGE_KERNEL_RO (__PP| 0| 0|___A|__NX| 0| 0|___G) > #define __PAGE_KERNEL_ROX (__PP| 0| 0|___A| 0| 0| 0|___G) > #define __PAGE_KERNEL (__PP|__RW| 0|___A|__NX|___D| 0|___G) > +#define __PAGE_KERNEL_NOGLOBAL (__PP|__RW| 0|___A|__NX|___D| 0| 0) > #define __PAGE_KERNEL_EXEC (__PP|__RW| 0|___A| 0|___D| 0|___G) > #define __PAGE_KERNEL_NOCACHE (__PP|__RW| 0|___A|__NX|___D| 0|___G| __NC) > #define __PAGE_KERNEL_VVAR (__PP| 0|_USR|___A|__NX| 0| 0|___G) > @@ -245,6 +246,7 @@ enum page_cache_mode { > #define __pgprot_mask(x) __pgprot((x) & __default_kernel_pte_mask) > > #define PAGE_KERNEL __pgprot_mask(__PAGE_KERNEL | _ENC) > +#define PAGE_KERNEL_NOGLOBAL __pgprot_mask(__PAGE_KERNEL_NOGLOBAL | _ENC) > #define PAGE_KERNEL_NOENC __pgprot_mask(__PAGE_KERNEL | 0) > #define PAGE_KERNEL_RO __pgprot_mask(__PAGE_KERNEL_RO | _ENC) > #define PAGE_KERNEL_EXEC __pgprot_mask(__PAGE_KERNEL_EXEC | _ENC) Should this be part of earlier mermap x86 patches? > diff --git a/mm/Kconfig b/mm/Kconfig > index e4cb52149acad..05b2bb841d0e0 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1506,7 +1506,14 @@ config MERMAP_KUNIT_TEST > If unsure, say N. > > config PAGE_ALLOC_UNMAPPED > - bool "Support allocating pages that aren't in the direct map" if COMPILE_TEST > - default COMPILE_TEST > + bool "Support allocating pages that aren't in the direct map" > + depends on MERMAP > + > +config PAGE_ALLOC_KUNIT_TESTS > + tristate "KUnit tests for the page allocator" if !KUNIT_ALL_TESTS > + depends on KUNIT > + default KUNIT_ALL_TESTS > + help > + Builds KUnit tests for the page allocator. This belongs to the next patch? > endmenu > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 710ee9f46d467..7c91dcbe32576 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -14,6 +14,7 @@ > * (lots of bits borrowed from Ingo Molnar & Andrew Morton) > */ > > +#include > #include > #include > #include > @@ -1327,15 +1328,72 @@ static inline bool should_skip_kasan_poison(struct page *page) > return page_kasan_tag(page) == KASAN_TAG_KERNEL; > } > > -static void kernel_init_pages(struct page *page, int numpages) > +#ifdef CONFIG_PAGE_ALLOC_UNMAPPED > +static inline bool pageblock_unmapped(struct page *page) > { > - int i; > + return freetype_flags(get_pageblock_freetype(page)) & FREETYPE_UNMAPPED; > +} > > - /* s390's use of memset() could override KASAN redzones. */ > - kasan_disable_current(); > - for (i = 0; i < numpages; i++) > - clear_highpage_kasan_tagged(page + i); > - kasan_enable_current(); > +static inline void clear_page_mermap(struct page *page, unsigned int numpages) > +{ > + void *mermap; > + > + BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHMEM)); > + > + /* Fast path: single mapping (may fail under preemption). */ > + mermap = mermap_get(page, numpages << PAGE_SHIFT, PAGE_KERNEL_NOGLOBAL); > + if (mermap) { > + void *buf = kasan_reset_tag(mermap_addr(mermap)); > + > + for (int i = 0; i < numpages; i++) > + clear_page(buf + (i << PAGE_SHIFT)); > + mermap_put(mermap); > + return; > + } > + > + /* Slow path, map each page individually (always succeeds). */ > + for (int i = 0; i < numpages; i++) { > + unsigned long flags; > + > + local_irq_save(flags); > + mermap = mermap_get_reserved(page + i, PAGE_KERNEL_NOGLOBAL); > + clear_page(kasan_reset_tag(mermap_addr(mermap))); > + mermap_put(mermap); > + local_irq_restore(flags); > + } > +} > +#else > +static inline bool pageblock_unmapped(struct page *page) > +{ > + return false; > +} > + > +static inline void clear_page_mermap(struct page *page, unsigned int numpages) > +{ > + BUG(); > +} > +#endif > + > +static void kernel_init_pages(struct page *page, unsigned int numpages) > +{ > + int num_blocks = DIV_ROUND_UP(numpages, pageblock_nr_pages); > + > + for (int block = 0; block < num_blocks; block++) { > + struct page *block_page = page + (block << pageblock_order); > + bool unmapped = pageblock_unmapped(block_page); > + > + /* s390's use of memset() could override KASAN redzones. */ > + kasan_disable_current(); > + if (unmapped) { > + clear_page_mermap(block_page, numpages); > + } else { > + for (int i = 0; i < min(numpages, pageblock_nr_pages); i++) > + clear_highpage_kasan_tagged(block_page + i); > + } > + kasan_enable_current(); > + > + numpages -= pageblock_nr_pages; > + } > } > > #ifdef CONFIG_MEM_ALLOC_PROFILING > @@ -5250,8 +5308,8 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order, > ac->nodemask = nodemask; > ac->freetype = gfp_freetype(gfp_mask); > > - /* Not implemented yet. */ > - if (freetype_flags(ac->freetype) & FREETYPE_UNMAPPED && gfp_mask & __GFP_ZERO) > + if (freetype_flags(ac->freetype) & FREETYPE_UNMAPPED && > + WARN_ON(!mermap_ready())) > return false; > > if (cpusets_enabled()) { >