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[80.230.25.45]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4903ad2ee77sm25394465e9.0.2026.05.21.07.09.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 May 2026 07:09:16 -0700 (PDT) Date: Thu, 21 May 2026 10:09:11 -0400 From: "Michael S. Tsirkin" To: linux-kernel@vger.kernel.org Cc: "David Hildenbrand (Arm)" , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , 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 Subject: Re: [PATCH v8 00/37] mm/virtio: skip redundant zeroing of host-zeroed pages Message-ID: <20260521100313-mutt-send-email-mst@kernel.org> References: Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: PkHafRtJl4J8dBh9rAUSqnkQC0Nw6RKmFwcEDcQPBe4_1779372558 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, May 20, 2026 at 06:20:13PM -0400, Michael S. Tsirkin wrote: > When a guest reports free pages to the hypervisor via virtio-balloon's > free page reporting, the host typically zeros those pages when reclaiming > their backing memory (e.g., via MADV_DONTNEED on anonymous mappings). > When the guest later reallocates those pages, the kernel zeros them > again, redundantly. FYI Sashiko reported a couple of issues. Mostly false positives but I tweaked commit log to make it clearer. But also a couple of real issues. So far I have this diff on top: ---> Changes from v8 to v9 candidate (code only): 1. mm/mempolicy.c (patch 1: mm: mempolicy: fix interleave index calculation) Combine vm_pgoff and VMA offset into a single expression before shifting, fixing carry loss for file-backed VMAs with unaligned vm_pgoff. 2. mm/memory-failure.c (patch 2: mm: memory-failure: serialize TestSetPageHWPoison with zone->lock) Wrap ClearPageHWPoison in retry path with zone->lock too. 3. mm/huge_memory.c (patch 19: mm: use __GFP_ZERO in vma_alloc_anon_folio_pmd) Fix stale comment: "folio_zero_user writes" -> "page zeroing". 4. mm/page_reporting.c (patch 4: mm: page_reporting: allow driver to set batch capacity) Drop rounddown_pow_of_two: compiler cannot optimize division by runtime variable anyway, and rounding halves batch size for non-power-of-2 capacity values. Commit log improvements (no code change): - Patch 1: rewritten to explain new single-expression formula (fixes patch 1) - Patch 2: mention ClearPageHWPoison in retry path (fixes patch 2) - Patch 4: drop "rounded down to a power of 2" (fixes patch 4) - Patch 10: "stub" wording simplified (fixes patch 10) - Patch 22: note PG_zeroed hint loss is harmless (fixes patch 22) - Patch 23: add PowerPC note (fixes patch 23) - Patch 28: explain DEVICE_INIT_ON_INFLATE is a follow-up (fixes patch 28) - Patch 29: explain flush over-reporting is by design (fixes patch 29) - Patch 33: note __SetPageZeroed is safe on frozen pages (fixes patch 33) - Patch 36: note __SetPageZeroed is safe on balloon-owned pages (fixes patch 36) --- diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 9845c920c29c..4978d34532ea 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1358,7 +1358,7 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma, /* * The memory barrier inside __folio_mark_uptodate makes sure that - * folio_zero_user writes become visible before the set_pmd_at() + * page zeroing becomes visible before the set_pmd_at() * write. */ __folio_mark_uptodate(folio); diff --git a/mm/memory-failure.c b/mm/memory-failure.c index a6b61172dd13..d106f2c135c7 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -2426,7 +2426,9 @@ int memory_failure(unsigned long pfn, int flags) } else { /* We lost the race, try again */ if (retry) { + spin_lock_irqsave(&zone->lock, mf_flags); ClearPageHWPoison(p); + spin_unlock_irqrestore(&zone->lock, mf_flags); retry = false; goto try_again; } diff --git a/mm/mempolicy.c b/mm/mempolicy.c index ea3043e0075b..f573ff32e94d 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -2048,9 +2048,9 @@ struct mempolicy *get_vma_policy(struct vm_area_struct *vma, pol = get_task_policy(current); if (pol->mode == MPOL_INTERLEAVE || pol->mode == MPOL_WEIGHTED_INTERLEAVE) { - *ilx += vma->vm_pgoff >> order; - *ilx += (addr >> (PAGE_SHIFT + order)) - - (vma->vm_start >> (PAGE_SHIFT + order)); + *ilx += (vma->vm_pgoff + + (addr >> PAGE_SHIFT) - + (vma->vm_start >> PAGE_SHIFT)) >> order; } return pol; } diff --git a/mm/page_reporting.c b/mm/page_reporting.c index 8b278a494ea5..3f584f538c68 100644 --- a/mm/page_reporting.c +++ b/mm/page_reporting.c @@ -443,9 +443,6 @@ int page_reporting_register(struct page_reporting_dev_info *prdev) if (!prdev->capacity || prdev->capacity > PAGE_REPORTING_CAPACITY) prdev->capacity = PAGE_REPORTING_CAPACITY; - /* Power of 2 so division by capacity in the budget calc is cheap */ - prdev->capacity = rounddown_pow_of_two(prdev->capacity); - /* initialize state and work structures */ atomic_set(&prdev->state, PAGE_REPORTING_IDLE); INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);