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[80.230.48.7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e6d895781sm104193005e9.0.2026.05.11.01.55.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2026 01:55:06 -0700 (PDT) Date: Mon, 11 May 2026 04:55:02 -0400 From: "Michael S. Tsirkin" To: linux-kernel@vger.kernel.org Cc: "David Hildenbrand (Arm)" , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , Muchun Song , Oscar Salvador , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Hugh Dickins , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Axel Rasmussen , Yuanchu Xie , Wei Xu , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Baoquan He , virtualization@lists.linux.dev, linux-mm@kvack.org, Andrea Arcangeli , "Liam R. Howlett" Subject: [PATCH v6 18/30] mm: page_reporting: skip redundant zeroing of host-zeroed reported pages Message-ID: <696f60c35b95b65fcb5872ae3f6ab6ffa194b555.1778488966.git.mst@redhat.com> References: Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: X-Mailer: git-send-email 2.27.0.106.g8ac3dc51b1 X-Mutt-Fcc: =sent X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NFyDWRQKhssL839AijFfW0mRk6sDNEeisaybeeZGfEM_1778489708 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline When a guest reports free pages to the hypervisor via the page reporting framework (used by virtio-balloon and hv_balloon), the host typically zeros those pages when reclaiming their backing memory. However, when those pages are later allocated in the guest, post_alloc_hook() unconditionally zeros them again if __GFP_ZERO is set. This double-zeroing is wasteful, especially for large pages. Avoid redundant zeroing: - Add a host_zeroes_pages flag to page_reporting_dev_info, allowing drivers to declare that their host zeros reported pages on reclaim. A static key (page_reporting_host_zeroes) gates the fast path. - Add PG_zeroed page flag (sharing PG_private bit) to mark pages that have been zeroed by the host. Set it in page_reporting_drain() after the host reports them. - Thread the zeroed bool through rmqueue -> prep_new_page -> post_alloc_hook, where it skips redundant zeroing for __GFP_ZERO allocations. No driver sets host_zeroes_pages yet; a follow-up patch to virtio_balloon is needed to opt in. Signed-off-by: Michael S. Tsirkin Assisted-by: Claude:claude-opus-4-6 Assisted-by: cursor-agent:GPT-5.4-xhigh --- include/linux/page-flags.h | 9 +++++ include/linux/page_reporting.h | 3 ++ mm/compaction.c | 6 ++-- mm/internal.h | 2 +- mm/page_alloc.c | 66 +++++++++++++++++++++++----------- mm/page_reporting.c | 14 +++++++- mm/page_reporting.h | 12 +++++++ 7 files changed, 87 insertions(+), 25 deletions(-) diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 0e03d816e8b9..4ee64134acc3 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -135,6 +135,8 @@ enum pageflags { PG_swapcache = PG_owner_priv_1, /* Swap page: swp_entry_t in private */ /* Some filesystems */ PG_checked = PG_owner_priv_1, + /* Page contents are known to be zero */ + PG_zeroed = PG_private, /* * Depending on the way an anonymous folio can be mapped into a page @@ -673,6 +675,13 @@ FOLIO_TEST_CLEAR_FLAG_FALSE(young) FOLIO_FLAG_FALSE(idle) #endif +/* + * PageZeroed() tracks pages known to be zero. The allocator + * uses this to skip redundant zeroing in post_alloc_hook(). + */ +__PAGEFLAG(Zeroed, zeroed, PF_NO_COMPOUND) +#define __PG_ZEROED (1UL << PG_zeroed) + /* * PageReported() is used to track reported free pages within the Buddy * allocator. We can use the non-atomic version of the test and set diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h index 5ab5be02fa15..c331c6b36687 100644 --- a/include/linux/page_reporting.h +++ b/include/linux/page_reporting.h @@ -14,6 +14,9 @@ struct page_reporting_dev_info { int (*report)(struct page_reporting_dev_info *prdev, struct scatterlist *sg, unsigned int nents); + /* If true, host zeros reported pages on reclaim */ + bool host_zeroes_pages; + /* work struct for processing reports */ struct delayed_work work; diff --git a/mm/compaction.c b/mm/compaction.c index 72684fe81e83..0471c5326ec0 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -82,7 +82,8 @@ static inline bool is_via_compact_memory(int order) { return false; } static struct page *mark_allocated_noprof(struct page *page, unsigned int order, gfp_t gfp_flags) { - post_alloc_hook(page, order, __GFP_MOVABLE, USER_ADDR_NONE); + __ClearPageZeroed(page); + post_alloc_hook(page, order, __GFP_MOVABLE, false, USER_ADDR_NONE); set_page_refcounted(page); return page; } @@ -1849,7 +1850,8 @@ static struct folio *compaction_alloc_noprof(struct folio *src, unsigned long da set_page_private(&freepage[size], start_order); } dst = (struct folio *)freepage; - post_alloc_hook(&dst->page, order, __GFP_MOVABLE, USER_ADDR_NONE); + __ClearPageZeroed(&dst->page); + post_alloc_hook(&dst->page, order, __GFP_MOVABLE, false, USER_ADDR_NONE); set_page_refcounted(&dst->page); if (order) prep_compound_page(&dst->page, order); diff --git a/mm/internal.h b/mm/internal.h index 751ae8911607..fa7ffea4d492 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -924,7 +924,7 @@ static inline void init_compound_tail(struct page *tail, } void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flags, - unsigned long user_addr); + bool zeroed, unsigned long user_addr); extern bool free_pages_prepare(struct page *page, unsigned int order); extern int user_min_free_kbytes; diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 76f39dd026ff..bd3b909cacdf 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1743,6 +1743,7 @@ static __always_inline void page_del_and_expand(struct zone *zone, bool was_reported = page_reported(page); __del_page_from_free_list(page, zone, high, migratetype); + nr_pages -= expand(zone, page, low, high, migratetype, was_reported); account_freepages(zone, -nr_pages, migratetype); } @@ -1815,8 +1816,10 @@ static inline bool should_skip_init(gfp_t flags) return (flags & __GFP_SKIP_ZERO); } + inline void post_alloc_hook(struct page *page, unsigned int order, - gfp_t gfp_flags, unsigned long user_addr) + gfp_t gfp_flags, bool zeroed, + unsigned long user_addr) { bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) && !should_skip_init(gfp_flags); @@ -1825,6 +1828,14 @@ inline void post_alloc_hook(struct page *page, unsigned int order, set_page_private(page, 0); + /* + * If the page is zeroed, skip memory initialization. + * We still need to handle tag zeroing separately since the host + * does not know about memory tags. + */ + if (zeroed && init && !zero_tags) + init = false; + arch_alloc_page(page, order); debug_pagealloc_map_pages(page, 1 << order); @@ -1882,13 +1893,13 @@ inline void post_alloc_hook(struct page *page, unsigned int order, } static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags, - unsigned int alloc_flags, - unsigned long user_addr) + unsigned int alloc_flags, bool zeroed, + unsigned long user_addr) { if (order && (gfp_flags & __GFP_COMP)) prep_compound_page(page, order); - post_alloc_hook(page, order, gfp_flags, user_addr); + post_alloc_hook(page, order, gfp_flags, zeroed, user_addr); /* * page is set pfmemalloc when ALLOC_NO_WATERMARKS was necessary to @@ -3154,6 +3165,7 @@ int __isolate_free_page(struct page *page, unsigned int order) } del_page_from_free_list(page, zone, order, mt); + __ClearPageZeroed(page); /* * Set the pageblock if the isolated page is at least half of a @@ -3226,7 +3238,7 @@ static inline void zone_statistics(struct zone *preferred_zone, struct zone *z, static __always_inline struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone, unsigned int order, unsigned int alloc_flags, - int migratetype) + int migratetype, bool *zeroed) { struct page *page; unsigned long flags; @@ -3261,6 +3273,8 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone, } } spin_unlock_irqrestore(&zone->lock, flags); + *zeroed = PageZeroed(page); + __ClearPageZeroed(page); } while (check_new_pages(page, order)); /* @@ -3329,10 +3343,9 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order) /* Remove page from the per-cpu list, caller must protect the list */ static inline struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order, - int migratetype, - unsigned int alloc_flags, + int migratetype, unsigned int alloc_flags, struct per_cpu_pages *pcp, - struct list_head *list) + struct list_head *list, bool *zeroed) { struct page *page; @@ -3367,6 +3380,8 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order, page = list_first_entry(list, struct page, pcp_list); list_del(&page->pcp_list); pcp->count -= 1 << order; + *zeroed = PageZeroed(page); + __ClearPageZeroed(page); } while (check_new_pages(page, order)); return page; @@ -3375,7 +3390,8 @@ struct page *__rmqueue_pcplist(struct zone *zone, unsigned int order, /* Lock and remove page from the per-cpu list */ static struct page *rmqueue_pcplist(struct zone *preferred_zone, struct zone *zone, unsigned int order, - int migratetype, unsigned int alloc_flags) + int migratetype, unsigned int alloc_flags, + bool *zeroed) { struct per_cpu_pages *pcp; struct list_head *list; @@ -3393,7 +3409,8 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone, */ pcp->free_count >>= 1; list = &pcp->lists[order_to_pindex(migratetype, order)]; - page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list); + page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, + pcp, list, zeroed); pcp_spin_unlock(pcp); if (page) { __count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order); @@ -3418,19 +3435,19 @@ static inline struct page *rmqueue(struct zone *preferred_zone, struct zone *zone, unsigned int order, gfp_t gfp_flags, unsigned int alloc_flags, - int migratetype) + int migratetype, bool *zeroed) { struct page *page; if (likely(pcp_allowed_order(order))) { page = rmqueue_pcplist(preferred_zone, zone, order, - migratetype, alloc_flags); + migratetype, alloc_flags, zeroed); if (likely(page)) goto out; } page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags, - migratetype); + migratetype, zeroed); out: /* Separate test+clear to avoid unnecessary atomics */ @@ -3821,6 +3838,7 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags, struct pglist_data *last_pgdat = NULL; bool last_pgdat_dirty_ok = false; bool no_fallback; + bool zeroed; bool skip_kswapd_nodes = nr_online_nodes > 1; bool skipped_kswapd_nodes = false; @@ -3965,10 +3983,11 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags, try_this_zone: page = rmqueue(zonelist_zone(ac->preferred_zoneref), zone, order, - gfp_mask, alloc_flags, ac->migratetype); + gfp_mask, alloc_flags, ac->migratetype, + &zeroed); if (page) { prep_new_page(page, order, gfp_mask, alloc_flags, - ac->user_addr); + zeroed, ac->user_addr); return page; } else { @@ -4195,9 +4214,11 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, count_vm_event(COMPACTSTALL); /* Prep a captured page if available */ - if (page) - prep_new_page(page, order, gfp_mask, alloc_flags, + if (page) { + __ClearPageZeroed(page); + prep_new_page(page, order, gfp_mask, alloc_flags, false, ac->user_addr); + } /* Try get a page from the freelist if available */ if (!page) @@ -5170,6 +5191,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, /* Attempt the batch allocation */ pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)]; while (nr_populated < nr_pages) { + bool zeroed = false; /* Skip existing pages */ if (page_array[nr_populated]) { @@ -5178,7 +5200,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, } page = __rmqueue_pcplist(zone, 0, ac.migratetype, alloc_flags, - pcp, pcp_list); + pcp, pcp_list, &zeroed); if (unlikely(!page)) { /* Try and allocate at least one page */ if (!nr_account) { @@ -5189,7 +5211,7 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid, } nr_account++; - prep_new_page(page, 0, gfp, 0, USER_ADDR_NONE); + prep_new_page(page, 0, gfp, 0, zeroed, USER_ADDR_NONE); set_page_refcounted(page); page_array[nr_populated++] = page; } @@ -6929,7 +6951,8 @@ static void split_free_frozen_pages(struct list_head *list, gfp_t gfp_mask) list_for_each_entry_safe(page, next, &list[order], lru) { int i; - post_alloc_hook(page, order, gfp_mask, USER_ADDR_NONE); + __ClearPageZeroed(page); + post_alloc_hook(page, order, gfp_mask, false, USER_ADDR_NONE); if (!order) continue; @@ -7134,8 +7157,9 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end, } else if (start == outer_start && end == outer_end && is_power_of_2(end - start)) { struct page *head = pfn_to_page(start); + __ClearPageZeroed(head); check_new_pages(head, order); - prep_new_page(head, order, gfp_mask, 0, USER_ADDR_NONE); + prep_new_page(head, order, gfp_mask, 0, false, USER_ADDR_NONE); } else { ret = -EINVAL; WARN(true, "PFN range: requested [%lu, %lu), allocated [%lu, %lu)\n", diff --git a/mm/page_reporting.c b/mm/page_reporting.c index 006f7cdddc18..37e4fce9eb38 100644 --- a/mm/page_reporting.c +++ b/mm/page_reporting.c @@ -50,6 +50,8 @@ EXPORT_SYMBOL_GPL(page_reporting_order); #define PAGE_REPORTING_DELAY (2 * HZ) static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly; +DEFINE_STATIC_KEY_FALSE(page_reporting_host_zeroes); + enum { PAGE_REPORTING_IDLE = 0, PAGE_REPORTING_REQUESTED, @@ -129,8 +131,11 @@ page_reporting_drain(struct page_reporting_dev_info *prdev, * report on the new larger page when we make our way * up to that higher order. */ - if (PageBuddy(page) && buddy_order(page) == order) + if (PageBuddy(page) && buddy_order(page) == order) { __SetPageReported(page); + if (page_reporting_host_zeroes_pages()) + __SetPageZeroed(page); + } } while ((sg = sg_next(sg))); /* reinitialize scatterlist now that it is empty */ @@ -391,6 +396,10 @@ int page_reporting_register(struct page_reporting_dev_info *prdev) /* Assign device to allow notifications */ rcu_assign_pointer(pr_dev_info, prdev); + /* enable zeroed page optimization if host zeroes reported pages */ + if (prdev->host_zeroes_pages) + static_branch_enable(&page_reporting_host_zeroes); + /* enable page reporting notification */ if (!static_key_enabled(&page_reporting_enabled)) { static_branch_enable(&page_reporting_enabled); @@ -415,6 +424,9 @@ void page_reporting_unregister(struct page_reporting_dev_info *prdev) /* Flush any existing work, and lock it out */ cancel_delayed_work_sync(&prdev->work); + + if (prdev->host_zeroes_pages) + static_branch_disable(&page_reporting_host_zeroes); } mutex_unlock(&page_reporting_mutex); diff --git a/mm/page_reporting.h b/mm/page_reporting.h index c51dbc228b94..736ea7b37e9e 100644 --- a/mm/page_reporting.h +++ b/mm/page_reporting.h @@ -15,6 +15,13 @@ DECLARE_STATIC_KEY_FALSE(page_reporting_enabled); extern unsigned int page_reporting_order; void __page_reporting_notify(void); +DECLARE_STATIC_KEY_FALSE(page_reporting_host_zeroes); + +static inline bool page_reporting_host_zeroes_pages(void) +{ + return static_branch_unlikely(&page_reporting_host_zeroes); +} + static inline bool page_reported(struct page *page) { return static_branch_unlikely(&page_reporting_enabled) && @@ -46,6 +53,11 @@ static inline void page_reporting_notify_free(unsigned int order) #else /* CONFIG_PAGE_REPORTING */ #define page_reported(_page) false +static inline bool page_reporting_host_zeroes_pages(void) +{ + return false; +} + static inline void page_reporting_notify_free(unsigned int order) { } -- MST