From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 510713CFF66 for ; Wed, 30 Sep 2026 19:01:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790794913; cv=none; b=LtSU5KlREcNnOyoSLFziGFFtOrXT310naYUyrMn+OFf4QlwsNSQFrQ+AE3/7geBwIh+fsffLkfLrwYfNOOnE60aOhb/inkF+Fd7lxbaGoZrV8hy3QNCa18RsUcx+I4/ZHp+5SPML8K/aMk8ZRVQDLQsAmK/l+oOHLxQDwl3HWms= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790794913; c=relaxed/simple; bh=bgkt9xZinXpkzOuIUWpKkrufFQMHB6knGPx2faJimMM=; h=Date:To:From:Subject:Message-Id; b=qIXTaziS6EOTRA7HJeFoJvoMC45g5bop3oOVNyyL+x7zJOmjIPvMnS3FdJXf8Bz3Yg4J5Px9ahXFZn/1NSVSSNhTkJG48LVos1A+gUy7HdRBPBX924W4HlvPHwvVLQ78f0vbt0Y+LADK9cq/mV/AJFHCkfAlC8KeDX7TCYdVrJk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=tcZRxGN1; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="tcZRxGN1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E3BF51F000FF; Wed, 30 Sep 2026 19:01:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1790794912; bh=LMo3yamaJG+RQ0R67rwQcDq4etH2QnXCB/E27kdCY7Q=; h=Date:To:From:Subject; b=tcZRxGN1iLMzeNqZSIZuUvSHiQHercoBwNmzmixATziHcHahBEek3P4/drOsGkDJq v761iPD/aybGY1nPlAehkMpOCjOfrs0TCVDGc3T4Agm8DUrNFotfMplOGkN4S+h1sZ WHo/BbZKkVt7GW5WxGfWF+E2KRw5ZVvAzFiF38rI= Date: Wed, 30 Sep 2026 12:01:51 -0700 To: mm-commits@vger.kernel.org,muchun.song@linux.dev,akpm@linux-foundation.org From: Andrew Morton Subject: + mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch added to mm-unstable branch Message-Id: <20260930190151.E3BF51F000FF@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages has been added to the -mm mm-unstable branch. Its filename is mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Muchun Song Subject: mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages Date: Wed, 30 Sep 2026 22:06:22 +0800 HugeTLB vmemmap optimization now uses per-zone shared tail vmemmap pages. Device DAX has not been switched to that mechanism yet. Switch device DAX to vmemmap_shared_tail_page() as well. This aligns DAX with HugeTLB by using the common per-zone shared tail vmemmap page. The optimization is enabled only for DEV-DAX through pgmap->vmemmap_shift, which supplies the compound page order recorded in section metadata before vmemmap population. Unlike FS-DAX, DEV-DAX does not modify tail struct pages, so sharing them is safe. Since the shared tail page can now back ZONE_DEVICE vmemmap mappings, initialize its entries with PG_reserved for device zones. Also skip poisoning vmemmap-optimizable sections while their struct pages may be shared. Each PTE mapping the shared device DAX tail page takes a page reference. A sufficiently large range could therefore cycle the reference count back to zero if population were allowed to continue after it became non-positive. Use try_get_page() so further mappings fail at that point. The section population error path tears down mappings created for the failed section, while the warning makes this currently impractical limit visible. Link: https://lore.kernel.org/20260930140627.57431-8-songmuchun@bytedance.com Signed-off-by: Muchun Song Signed-off-by: Andrew Morton Acked-by: Qi Zheng Cc: David Hildenbrand Cc: Oscar Salvador Cc: Madhavan Srinivasan Cc: Michael Ellerman Cc: Jonathan Corbet Cc: Lorenzo Stoakes Cc: Mike Rapoport Cc: Nicholas Piggin Cc: Christophe Leroy Cc: Ritesh Harjani (IBM) Cc: Shrikanth Hegde Cc: Randy Dunlap Cc: Lance Yang --- include/linux/mmzone.h | 10 ++++++ mm/memory_hotplug.c | 6 ++- mm/sparse-vmemmap.c | 63 ++++++++++++++++----------------------- 3 files changed, 40 insertions(+), 39 deletions(-) --- a/include/linux/mmzone.h~mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages +++ a/include/linux/mmzone.h @@ -2143,11 +2143,21 @@ static inline int online_device_section( return section && ((section->section_mem_map & flags) == flags); } + +static inline struct zone *device_zone(int nid) +{ + return &NODE_DATA(nid)->node_zones[ZONE_DEVICE]; +} #else static inline int online_device_section(const struct mem_section *section) { return 0; } + +static inline struct zone *device_zone(int nid) +{ + return NULL; +} #endif static inline int online_section_nr(unsigned long nr) --- a/mm/memory_hotplug.c~mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages +++ a/mm/memory_hotplug.c @@ -43,6 +43,7 @@ #include "mm_init.h" #include "page_alloc.h" #include "shuffle.h" +#include "sparse.h" enum { MEMMAP_ON_MEMORY_DISABLE = 0, @@ -554,8 +555,9 @@ void remove_pfn_range_from_zone(struct z /* Select all remaining pages up to the next section boundary */ cur_nr_pages = min(end_pfn - pfn, SECTION_ALIGN_UP(pfn + 1) - pfn); - page_init_poison(pfn_to_page(pfn), - sizeof(struct page) * cur_nr_pages); + if (!section_vmemmap_optimizable(__pfn_to_section(pfn))) + page_init_poison(pfn_to_page(pfn), + sizeof(struct page) * cur_nr_pages); } /* --- a/mm/sparse-vmemmap.c~mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages +++ a/mm/sparse-vmemmap.c @@ -225,6 +225,8 @@ struct page __ref *vmemmap_shared_tail_p set_page_node(page, zone_to_nid(zone)); set_page_zone(page, zone_idx(zone)); prep_compound_tail(page, NULL, order); + if (zone_is_zone_device(zone)) + __SetPageReserved(page); } page = virt_to_page(addr); @@ -288,14 +290,18 @@ static pte_t * __meminit vmemmap_pte_pop /* * When a PTE/PMD entry is freed from the init_mm * there's a free_pages() call to this page allocated - * above. Thus this get_page() is paired with the + * above. Thus this try_get_page() is paired with the * put_page_testzero() on the freeing path. * This can only called by certain ZONE_DEVICE path, * and through vmemmap_populate_compound_pages() when * slab is available. + * + * Use try_get_page() to prevent the shared page refcount + * from overflowing. */ - if (flags & VMEMMAP_POPULATE_DAX) - get_page(pfn_to_page(ptpfn)); + if ((flags & VMEMMAP_POPULATE_DAX) && + !try_get_page(pfn_to_page(ptpfn))) + return NULL; } entry = pfn_pte(ptpfn, PAGE_KERNEL); set_pte_at(&init_mm, addr, pte, entry); @@ -529,47 +535,27 @@ static bool __meminit reuse_compound_sec return !IS_ALIGNED(offset, nr_pages) && nr_pages > PAGES_PER_SUBSECTION; } -static pte_t * __meminit compound_section_tail_page(unsigned long addr) -{ - pte_t *pte; - - addr -= PAGE_SIZE; - - /* - * Assuming sections are populated sequentially, the previous section's - * page data can be reused. - */ - pte = pte_offset_kernel(pmd_off_k(addr), addr); - if (!pte) - return NULL; - - return pte; -} - static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn, unsigned long start, unsigned long end, int node, struct dev_pagemap *pgmap) { const unsigned long flags = VMEMMAP_POPULATE_DAX; + const unsigned int order = pfn_to_section_compound_order(start_pfn); unsigned long size, addr; pte_t *pte; + struct page *page; int rc; - if (reuse_compound_section(start_pfn, pgmap)) { - pte = compound_section_tail_page(start); - if (!pte) - return -ENOMEM; + page = vmemmap_shared_tail_page(order, device_zone(node)); + if (!page) + return -ENOMEM; - /* - * Reuse the page that was populated in the prior iteration - * with just tail struct pages. - */ + if (reuse_compound_section(start_pfn, pgmap)) return vmemmap_populate_range(start, end, node, NULL, - pte_pfn(ptep_get(pte)), flags); - } + page_to_pfn(page), flags); - size = min(end - start, pgmap_vmemmap_nr(pgmap) * sizeof(struct page)); + size = min(end - start, (1UL << order) * sizeof(struct page)); for (addr = start; addr < end; addr += size) { unsigned long next, last = addr + size; @@ -585,12 +571,12 @@ static int __meminit vmemmap_populate_co return -ENOMEM; /* - * Reuse the previous page for the rest of tail pages + * Reuse the shared page for the rest of tail pages * See layout diagram in Documentation/mm/vmemmap_dedup.rst */ next += PAGE_SIZE; rc = vmemmap_populate_range(next, last, node, NULL, - pte_pfn(ptep_get(pte)), flags); + page_to_pfn(page), flags); if (rc) return -ENOMEM; } @@ -922,13 +908,16 @@ int __meminit sparse_add_section(int nid if (IS_ERR(memmap)) return PTR_ERR(memmap); + ms = __nr_to_section(section_nr); /* - * Poison uninitialized struct pages in order to catch invalid flags - * combinations. + * Poison uninitialized struct pages to catch invalid flag combinations. + * + * Tail struct pages in a vmemmap-optimized section are initialized and + * shared during vmemmap population, so they must not be overwritten here. */ - page_init_poison(memmap, sizeof(struct page) * nr_pages); + if (!section_vmemmap_optimizable(ms)) + page_init_poison(memmap, sizeof(struct page) * nr_pages); - ms = __nr_to_section(section_nr); __section_mark_present(ms, section_nr); /* Align memmap to section boundary in the subsection case */ _ Patches currently in -mm which might be from muchun.song@linux.dev are mm-sparse-vmemmap-factor-out-shared-vmemmap-tail-page-allocation.patch mm-sparse-vmemmap-allocate-shared-tail-page-array-dynamically.patch mm-sparse-vmemmap-introduce-config_vmemmap_optimization.patch mm-sparse-vmemmap-open-code-init_compound_tail.patch mm-sparse-vmemmap-prepare-dax-vmemmap-population-for-compound-page-orders.patch mm-sparse-vmemmap-set-compound-page-order-for-device-dax.patch mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages.patch mm-sparse-vmemmap-switch-device-dax-to-shared-tail-vmemmap-pages-fix.patch mm-sparse-vmemmap-move-vmemmap-optimization-helpers-to-a-public-header.patch powerpc-mm-switch-device-dax-to-shared-tail-vmemmap-pages.patch powerpc-mm-switch-device-dax-to-shared-tail-vmemmap-pages-fix.patch mm-sparse-vmemmap-drop-the-extra-tail-page-from-device-dax-reservation.patch mm-sparse-vmemmap-drop-unused-section_nr_vmemmap_pages-arguments.patch documentation-mm-update-dax-vmemmap-deduplication-docs.patch mm-sparse-vmemmap-drop-vmemmap_populate_dax.patch mm-sparse-vmemmap-support-device-dax-in-common-vmemmap-path.patch mm-sparse-vmemmap-drop-device-dax-specific-population-path.patch mm-sparse-vmemmap-remove-the-unused-ptpfn-argument.patch mm-sparse-vmemmap-open-code-vmemmap_populate_address.patch mm-mm_init-add-zone-mismatch-warning-during-page-init.patch