kernel/linux.git/mm/rmap.c, branch v6.18.21

mm/huge_memory: fix early failure try_to_migrate() when split huge pmd for shared THP

2026-03-25T10:10:30+00:00

commit 939080834fef3ce42fdbcfef33fd29c9ffe5bbed upstream. Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") return false unconditionally after split_huge_pmd_locked(). This may fail try_to_migrate() early when TTU_SPLIT_HUGE_PMD is specified. The reason is the above commit adjusted try_to_migrate_one() to, when a PMD-mapped THP entry is found, and TTU_SPLIT_HUGE_PMD is specified (for example, via unmap_folio()), return false unconditionally. This breaks the rmap walk and fail try_to_migrate() early, if this PMD-mapped THP is mapped in multiple processes. The user sensible impact of this bug could be: * On memory pressure, shrink_folio_list() may split partially mapped folio with split_folio_to_list(). Then free unmapped pages without IO. If failed, it may not be reclaimed. * On memory failure, memory_failure() would call try_to_split_thp_page() to split folio contains the bad page. If succeed, the PG_has_hwpoisoned bit is only set in the after-split folio contains @split_at. By doing so, we limit bad memory. If failed to split, the whole folios is not usable. One way to reproduce: Create an anonymous THP range and fork 512 children, so we have a THP shared mapped in 513 processes. Then trigger folio split with /sys/kernel/debug/split_huge_pages debugfs to split the THP folio to order 0. Without the above commit, we can successfully split to order 0. With the above commit, the folio is still a large folio. And currently there are two core users of TTU_SPLIT_HUGE_PMD: * try_to_unmap_one() * try_to_migrate_one() try_to_unmap_one() would restart the rmap walk, so only try_to_migrate_one() is affected. We can't simply revert commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()"), since it removed some duplicated check covered by page_vma_mapped_walk(). This patch fixes this by restart page_vma_mapped_walk() after split_huge_pmd_locked(). Since we cannot simply return "true" to fix the problem, as that would affect another case: When invoking folio_try_share_anon_rmap_pmd() from split_huge_pmd_locked(), the latter can fail and leave a large folio mapped through PTEs, in which case we ought to return true from try_to_migrate_one(). This might result in unnecessary walking of the rmap but is relatively harmless. Link: https://lkml.kernel.org/r/20260305015006.27343-1-richard.weiyang@gmail.com Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and split_huge_pmd_locked()") Signed-off-by: Wei Yang Reviewed-by: Baolin Wang Reviewed-by: Zi Yan Tested-by: Lance Yang Reviewed-by: Lance Yang Reviewed-by: Gavin Guo Acked-by: David Hildenbrand (arm) Reviewed-by: Lorenzo Stoakes (Oracle) Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

mm/rmap: fix incorrect pte restoration for lazyfree folios

2026-03-25T10:10:30+00:00

commit 29f40594a28114b9a9bc87f6cf7bbee9609628f2 upstream. We batch unmap anonymous lazyfree folios by folio_unmap_pte_batch. If the batch has a mix of writable and non-writable bits, we may end up setting the entire batch writable. Fix this by respecting writable bit during batching. Although on a successful unmap of a lazyfree folio, the soft-dirty bit is lost, preserve it on pte restoration by respecting the bit during batching, to make the fix consistent w.r.t both writable bit and soft-dirty bit. I was able to write the below reproducer and crash the kernel. Explanation of reproducer (set 64K mTHP to always): Fault in a 64K large folio. Split the VMA at mid-point with MADV_DONTFORK. fork() - parent points to the folio with 8 writable ptes and 8 non-writable ptes. Merge the VMAs with MADV_DOFORK so that folio_unmap_pte_batch() can determine all the 16 ptes as a batch. Do MADV_FREE on the range to mark the folio as lazyfree. Write to the memory to dirty the pte, eventually rmap will dirty the folio. Then trigger reclaim, we will hit the pte restoration path, and the kernel will crash with the trace given below. The BUG happens at: BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw); The code path is asking for anonymous page to be mapped writable into the pagetable. The BUG_ON() firing implies that such a writable page has been mapped into the pagetables of more than one process, which breaks anonymous memory/CoW semantics. [ 21.134473] kernel BUG at mm/page_table_check.c:118! [ 21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [ 21.135917] Modules linked in: [ 21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT [ 21.136858] Hardware name: linux,dummy-virt (DT) [ 21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) [ 21.137308] pc : page_table_check_set+0x28c/0x2a8 [ 21.137607] lr : page_table_check_set+0x134/0x2a8 [ 21.137885] sp : ffff80008a3b3340 [ 21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000 [ 21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001 [ 21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30 [ 21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000 [ 21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff [ 21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020 [ 21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0 [ 21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff [ 21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002 [ 21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0 [ 21.141991] Call trace: [ 21.142093] page_table_check_set+0x28c/0x2a8 (P) [ 21.142265] __page_table_check_ptes_set+0x144/0x1e8 [ 21.142441] __set_ptes_anysz.constprop.0+0x160/0x1a8 [ 21.142766] contpte_set_ptes+0xe8/0x140 [ 21.142907] try_to_unmap_one+0x10c4/0x10d0 [ 21.143177] rmap_walk_anon+0x100/0x250 [ 21.143315] try_to_unmap+0xa0/0xc8 [ 21.143441] shrink_folio_list+0x59c/0x18a8 [ 21.143759] shrink_lruvec+0x664/0xbf0 [ 21.144043] shrink_node+0x218/0x878 [ 21.144285] __node_reclaim.constprop.0+0x98/0x338 [ 21.144763] user_proactive_reclaim+0x2a4/0x340 [ 21.145056] reclaim_store+0x3c/0x60 [ 21.145216] dev_attr_store+0x20/0x40 [ 21.145585] sysfs_kf_write+0x84/0xa8 [ 21.145835] kernfs_fop_write_iter+0x130/0x1c8 [ 21.145994] vfs_write+0x2b8/0x368 [ 21.146119] ksys_write+0x70/0x110 [ 21.146240] __arm64_sys_write+0x24/0x38 [ 21.146380] invoke_syscall+0x50/0x120 [ 21.146513] el0_svc_common.constprop.0+0x48/0xf8 [ 21.146679] do_el0_svc+0x28/0x40 [ 21.146798] el0_svc+0x34/0x110 [ 21.146926] el0t_64_sync_handler+0xa0/0xe8 [ 21.147074] el0t_64_sync+0x198/0x1a0 [ 21.147225] Code: f9400441 b4fff241 17ffff94 d4210000 (d4210000) [ 21.147440] ---[ end trace 0000000000000000 ]--- #define _GNU_SOURCE #include #include #include #include #include #include #include #include void write_to_reclaim() { const char *path = "/sys/devices/system/node/node0/reclaim"; const char *value = "409600000000"; int fd = open(path, O_WRONLY); if (fd == -1) { perror("open"); exit(EXIT_FAILURE); } if (write(fd, value, sizeof("409600000000") - 1) == -1) { perror("write"); close(fd); exit(EXIT_FAILURE); } printf("Successfully wrote %s to %s\n", value, path); close(fd); } int main() { char *ptr = mmap((void *)(1UL << 30), 1UL << 16, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if ((unsigned long)ptr != (1UL << 30)) { perror("mmap"); return 1; } /* a 64K folio gets faulted in */ memset(ptr, 0, 1UL << 16); /* 32K half will not be shared into child */ if (madvise(ptr, 1UL << 15, MADV_DONTFORK)) { perror("madvise madv dontfork"); return 1; } pid_t pid = fork(); if (pid < 0) { perror("fork"); return 1; } else if (pid == 0) { sleep(15); } else { /* merge VMAs. now first half of the 16 ptes are writable, the other half not. */ if (madvise(ptr, 1UL << 15, MADV_DOFORK)) { perror("madvise madv fork"); return 1; } if (madvise(ptr, (1UL << 16), MADV_FREE)) { perror("madvise madv free"); return 1; } /* dirty the large folio */ (*ptr) += 10; write_to_reclaim(); // sleep(10); waitpid(pid, NULL, 0); } } Link: https://lkml.kernel.org/r/20260303061528.2429162-1-dev.jain@arm.com Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") Signed-off-by: Dev Jain Acked-by: David Hildenbrand (Arm) Reviewed-by: Lorenzo Stoakes Reviewed-by: Barry Song Reviewed-by: Wei Yang Tested-by: Lance Yang Cc: Anshuman Khandual Cc: Harry Yoo Cc: Jann Horn Cc: Liam Howlett Cc: Rik van Riel Cc: Ryan Roberts Cc: Vlastimil Babka Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather

2026-02-19T15:31:34+00:00

commit 8ce720d5bd91e9dc16db3604aa4b1bf76770a9a1 upstream. As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race") we can end up in some situations where we perform so many IPI broadcasts when unsharing hugetlb PMD page tables that it severely regresses some workloads. In particular, when we fork()+exit(), or when we munmap() a large area backed by many shared PMD tables, we perform one IPI broadcast per unshared PMD table. There are two optimizations to be had: (1) When we process (unshare) multiple such PMD tables, such as during exit(), it is sufficient to send a single IPI broadcast (as long as we respect locking rules) instead of one per PMD table. Locking prevents that any of these PMD tables could get reused before we drop the lock. (2) When we are not the last sharer (> 2 users including us), there is no need to send the IPI broadcast. The shared PMD tables cannot become exclusive (fully unshared) before an IPI will be broadcasted by the last sharer. Concurrent GUP-fast could walk into a PMD table just before we unshared it. It could then succeed in grabbing a page from the shared page table even after munmap() etc succeeded (and supressed an IPI). But there is not difference compared to GUP-fast just sleeping for a while after grabbing the page and re-enabling IRQs. Most importantly, GUP-fast will never walk into page tables that are no-longer shared, because the last sharer will issue an IPI broadcast. (if ever required, checking whether the PUD changed in GUP-fast after grabbing the page like we do in the PTE case could handle this) So let's rework PMD sharing TLB flushing + IPI sync to use the mmu_gather infrastructure so we can implement these optimizations and demystify the code at least a bit. Extend the mmu_gather infrastructure to be able to deal with our special hugetlb PMD table sharing implementation. To make initialization of the mmu_gather easier when working on a single VMA (in particular, when dealing with hugetlb), provide tlb_gather_mmu_vma(). We'll consolidate the handling for (full) unsharing of PMD tables in tlb_unshare_pmd_ptdesc() and tlb_flush_unshared_tables(), and track in "struct mmu_gather" whether we had (full) unsharing of PMD tables. Because locking is very special (concurrent unsharing+reuse must be prevented), we disallow deferring flushing to tlb_finish_mmu() and instead require an explicit earlier call to tlb_flush_unshared_tables(). From hugetlb code, we call huge_pmd_unshare_flush() where we make sure that the expected lock protecting us from concurrent unsharing+reuse is still held. Check with a VM_WARN_ON_ONCE() in tlb_finish_mmu() that tlb_flush_unshared_tables() was properly called earlier. Document it all properly. Notes about tlb_remove_table_sync_one() interaction with unsharing: There are two fairly tricky things: (1) tlb_remove_table_sync_one() is a NOP on architectures without CONFIG_MMU_GATHER_RCU_TABLE_FREE. Here, the assumption is that the previous TLB flush would send an IPI to all relevant CPUs. Careful: some architectures like x86 only send IPIs to all relevant CPUs when tlb->freed_tables is set. The relevant architectures should be selecting MMU_GATHER_RCU_TABLE_FREE, but x86 might not do that in stable kernels and it might have been problematic before this patch. Also, the arch flushing behavior (independent of IPIs) is different when tlb->freed_tables is set. Do we have to enlighten them to also take care of tlb->unshared_tables? So far we didn't care, so hopefully we are fine. Of course, we could be setting tlb->freed_tables as well, but that might then unnecessarily flush too much, because the semantics of tlb->freed_tables are a bit fuzzy. This patch changes nothing in this regard. (2) tlb_remove_table_sync_one() is not a NOP on architectures with CONFIG_MMU_GATHER_RCU_TABLE_FREE that actually don't need a sync. Take x86 as an example: in the common case (!pv, !X86_FEATURE_INVLPGB) we still issue IPIs during TLB flushes and don't actually need the second tlb_remove_table_sync_one(). This optimized can be implemented on top of this, by checking e.g., in tlb_remove_table_sync_one() whether we really need IPIs. But as described in (1), it really must honor tlb->freed_tables then to send IPIs to all relevant CPUs. Notes on TLB flushing changes: (1) Flushing for non-shared PMD tables We're converting from flush_hugetlb_tlb_range() to tlb_remove_huge_tlb_entry(). Given that we properly initialize the MMU gather in tlb_gather_mmu_vma() to be hugetlb aware, similar to __unmap_hugepage_range(), that should be fine. (2) Flushing for shared PMD tables We're converting from various things (flush_hugetlb_tlb_range(), tlb_flush_pmd_range(), flush_tlb_range()) to tlb_flush_pmd_range(). tlb_flush_pmd_range() achieves the same that tlb_remove_huge_tlb_entry() would achieve in these scenarios. Note that tlb_remove_huge_tlb_entry() also calls __tlb_remove_tlb_entry(), however that is only implemented on powerpc, which does not support PMD table sharing. Similar to (1), tlb_gather_mmu_vma() should make sure that TLB flushing keeps on working as expected. Further, note that the ptdesc_pmd_pts_dec() in huge_pmd_share() is not a concern, as we are holding the i_mmap_lock the whole time, preventing concurrent unsharing. That ptdesc_pmd_pts_dec() usage will be removed separately as a cleanup later. There are plenty more cleanups to be had, but they have to wait until this is fixed. [david@kernel.org: fix kerneldoc] Link: https://lkml.kernel.org/r/f223dd74-331c-412d-93fc-69e360a5006c@kernel.org Link: https://lkml.kernel.org/r/20251223214037.580860-5-david@kernel.org Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race") Signed-off-by: David Hildenbrand (Red Hat) Reported-by: "Uschakow, Stanislav" Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/ Tested-by: Laurence Oberman Acked-by: Harry Yoo Reviewed-by: Lorenzo Stoakes Cc: Lance Yang Cc: Liu Shixin Cc: Oscar Salvador Cc: Rik van Riel Cc: Signed-off-by: Andrew Morton Signed-off-by: David Hildenbrand (Arm) Signed-off-by: Greg Kroah-Hartman

mm/rmap: fix two comments related to huge_pmd_unshare()

2026-01-30T09:32:15+00:00

commit a8682d500f691b6dfaa16ae1502d990aeb86e8be upstream. PMD page table unsharing no longer touches the refcount of a PMD page table. Also, it is not about dropping the refcount of a "PMD page" but the "PMD page table". Let's just simplify by saying that the PMD page table was unmapped, consequently also unmapping the folio that was mapped into this page. This code should be deduplicated in the future. Link: https://lkml.kernel.org/r/20251223214037.580860-4-david@kernel.org Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: David Hildenbrand (Red Hat) Reviewed-by: Rik van Riel Tested-by: Laurence Oberman Reviewed-by: Lorenzo Stoakes Acked-by: Oscar Salvador Cc: Liu Shixin Cc: Harry Yoo Cc: Lance Yang Cc: "Uschakow, Stanislav" Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

mm/rmap: improve mlock tracking for large folios

2025-09-28T18:51:31+00:00

The kernel currently does not mlock large folios when adding them to rmap, stating that it is difficult to confirm that the folio is fully mapped and safe to mlock it. This leads to a significant undercount of Mlocked in /proc/meminfo, causing problems in production where the stat was used to estimate system utilization and determine if load shedding is required. However, nowadays the caller passes a number of pages of the folio that are getting mapped, making it easy to check if the entire folio is mapped to the VMA. mlock the folio on rmap if it is fully mapped to the VMA. Mlocked in /proc/meminfo can still undercount, but the value is closer the truth and is useful for userspace. Link: https://lkml.kernel.org/r/20250923110711.690639-7-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau Acked-by: David Hildenbrand Acked-by: Johannes Weiner Acked-by: Shakeel Butt Reviewed-by: Lorenzo Stoakes Reviewed-by: Baolin Wang Signed-off-by: Andrew Morton

mm/rmap: mlock large folios in try_to_unmap_one()

2025-09-28T18:51:30+00:00

Currently, try_to_unmap_once() only tries to mlock small folios. Use logic similar to folio_referenced_one() to mlock large folios: only do this for fully mapped folios and under page table lock that protects all page table entries. [akpm@linux-foundation.org: s/CROSSSED/CROSSED/] Link: https://lkml.kernel.org/r/20250923110711.690639-4-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau Reviewed-by: Shakeel Butt Cc: Baolin Wang Cc: David Hildenbrand Cc: Johannes Weiner Cc: Lorenzo Stoakes Signed-off-by: Andrew Morton

mm/rmap: fix a mlock race condition in folio_referenced_one()

2025-09-28T18:51:30+00:00

The mlock_vma_folio() function requires the page table lock to be held in order to safely mlock the folio. However, folio_referenced_one() mlocks a large folios outside of the page_vma_mapped_walk() loop where the page table lock has already been dropped. Rework the mlock logic to use the same code path inside the loop for both large and small folios. Use PVMW_PGTABLE_CROSSED to detect when the folio is mapped across a page table boundary. [akpm@linux-foundation.org: s/CROSSSED/CROSSED/] Link: https://lkml.kernel.org/r/20250923110711.690639-3-kirill@shutemov.name Signed-off-by: Kiryl Shutsemau Reviewed-by: Shakeel Butt Cc: Baolin Wang Cc: David Hildenbrand Cc: Johannes Weiner Cc: Lorenzo Stoakes Signed-off-by: Andrew Morton

mm/rmap: use folio_large_nr_pages() when we are sure it is a large folio

2025-09-13T23:55:15+00:00

Non-large folio is handled at the beginning, so it is a large folio for sure. Use folio_large_nr_pages() here like elsewhere. Link: https://lkml.kernel.org/r/20250817032647.29147-3-richard.weiyang@gmail.com Signed-off-by: Wei Yang Reviewed-by: Lorenzo Stoakes Acked-by: David Hildenbrand Cc: Rik van Riel Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Harry Yoo Signed-off-by: Andrew Morton

mm/rmap: not necessary to mask off FOLIO_PAGES_MAPPED

2025-09-13T23:55:14+00:00

At this point, we are in an if branch conditional on (nr < ENTIRELY_MAPPED), and FOLIO_PAGES_MAPPED is equal to (ENTIRELY_MAPPED - 1). This means the upper bits are already cleared. It is not necessary to mask it off. Link: https://lkml.kernel.org/r/20250817032647.29147-2-richard.weiyang@gmail.com Signed-off-by: Wei Yang Reviewed-by: Lorenzo Stoakes Acked-by: David Hildenbrand Cc: Rik van Riel Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Harry Yoo Signed-off-by: Andrew Morton

mm, x86/mm: move creating the tlb_flush event back to x86 code

2025-09-13T23:55:14+00:00

Commit e73ad5ff2f76 ("mm, x86/mm: Make the batched unmap TLB flush API more generic") moved the trace_tlb_flush out of mm/rmap.c and back into x86 specific architecture, but it kept the include to the events/tlb.h file, even though it didn't use that event. Then another commit came in and added more events to the mm/rmap.c file and moved the #define CREATE_TRACE_POINTS define from the x86 specific architecture to the generic mm/rmap.h file to create both the tlb_flush tracepoint and the new tracepoints. But since the tlb_flush tracepoint is only x86 specific, it now creates that tracepoint for all other architectures and this wastes approximately 5K of text and meta data that will not be used. Remove the events/tlb.h from mm/rmap.c and add the define CREATE_TRACE_POINTS back in the x86 code. Link: https://lkml.kernel.org/r/20250612100313.3b9a8b80@batman.local.home Fixes: e73ad5ff2f76 ("mm, x86/mm: Make the batched unmap TLB flush API more generic") Signed-off-by: Steven Rostedt (Google) Reviewed-by: Lorenzo Stoakes Cc: Andy Lutomirski Cc: Borislav Betkov Cc: David Hildenbrand Cc: "Masami Hiramatsu (Google)" Cc: Mathieu Desnoyers Cc: Peter Zijlstra Cc: Thomas Gleinxer Signed-off-by: Andrew Morton