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b=GDknwJ8HcRhOqT37R1NBcIhBpefZ/P+to6KKEA0paxo4VkGwimSYx6qTHkolCsohC WfJ4LYZ3vXPe9GWTBsMwzhjT1cRS50S1uKI0v5yAh50szYhQjQktb8rdVW7ytpKW5t 2VVpNkWG0xC0V/O9n/B5DeAoFVBI5tc+0mbQ/QBjp3AIpVSWC/emdHV1BxkFVa++O5 TF13GzxzY0n+C7FtMsmHNNmVxhUW/dMiSYVxmyvhMM+sEbLvKYY8/M7kKk79WmUQe+ S7NMZKVQwTzeLHeLng7U8TdG8rd6gxsaVhRL8sMhWErrQrfIVgd3zXKGX2AivGUook QBWCnRTHIy8Wg== From: "David Hildenbrand (Arm)" To: stable@vger.kernel.org Cc: linux-mm@kvack.org, Jane Chu , Harry Yoo , Oscar Salvador , David Hildenbrand , Jann Horn , Liu Shixin , Muchun Song , Andrew Morton , David Hildenbrand Subject: [PATCH 5.15.y 2/6] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count Date: Wed, 18 Feb 2026 12:01:25 +0100 Message-ID: <20260218110129.41578-3-david@kernel.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260218110129.41578-1-david@kernel.org> References: <2026012608-tulip-moisten-c6f6@gregkh> <20260218110129.41578-1-david@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 9D9F11A0013 X-Stat-Signature: tzwhde7t19t1ums7jsw3uob68int53dz X-HE-Tag: 1771412503-611311 X-HE-Meta: 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 H11kGnsv LLmr5i3zc+4IRObb+s+RcfTUM8TptWrw105nVaJ0As6HOrVF0NSVuBxUkvizpaTsRtYjNgsjPAay0COf4ouoKeUdsG3TcaJfT/fVhGcaMBWLVErmkGjVzX11Th9qjOq/MaSxpAZgWiLG1pgHCb8IZGMH94vDdyNII+Ne+3dL4j8haR9/ITUnP15mjBPw1TkFIhk/vebUIhry0jh4yvC9uj5fURR7eF6b2OrrBhAE/vpG06l/qS17UQtnd+fVUtpa4jArRCrN/0i9aBYL23FseK7xYnjNbdFSTZESFsHaOO9MHY3vM8d5WkHvozO6uU5R6yQOa4R4MEFw31G3wPD1tNdH4vDycn3rES9TkgAgLbPPXVs2IlqvXqBKdGvVAq8IxmiAdjzrQEgo6H8azpxY39Jn6GKLuzSqCfNofQqXBnS3aowY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Jane Chu commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") introduced ->pt_share_count dedicated to hugetlb PMD share count tracking, but omitted fixing copy_hugetlb_page_range(), leaving the function relying on page_count() for tracking that no longer works. When lazy page table copy for hugetlb is disabled, that is, revert commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()") fork()'ing with hugetlb PMD sharing quickly lockup - [ 239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s! [ 239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0 [ 239.446631] Call Trace: [ 239.446633] [ 239.446636] _raw_spin_lock+0x3f/0x60 [ 239.446639] copy_hugetlb_page_range+0x258/0xb50 [ 239.446645] copy_page_range+0x22b/0x2c0 [ 239.446651] dup_mmap+0x3e2/0x770 [ 239.446654] dup_mm.constprop.0+0x5e/0x230 [ 239.446657] copy_process+0xd17/0x1760 [ 239.446660] kernel_clone+0xc0/0x3e0 [ 239.446661] __do_sys_clone+0x65/0xa0 [ 239.446664] do_syscall_64+0x82/0x930 [ 239.446668] ? count_memcg_events+0xd2/0x190 [ 239.446671] ? syscall_trace_enter+0x14e/0x1f0 [ 239.446676] ? syscall_exit_work+0x118/0x150 [ 239.446677] ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0 [ 239.446681] ? clear_bhb_loop+0x30/0x80 [ 239.446684] ? clear_bhb_loop+0x30/0x80 [ 239.446686] entry_SYSCALL_64_after_hwframe+0x76/0x7e There are two options to resolve the potential latent issue: 1. warn against PMD sharing in copy_hugetlb_page_range(), 2. fix it. This patch opts for the second option. While at it, simplify the comment, the details are not actually relevant anymore. Link: https://lkml.kernel.org/r/20250916004520.1604530-1-jane.chu@oracle.com Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: Jane Chu Reviewed-by: Harry Yoo Acked-by: Oscar Salvador Acked-by: David Hildenbrand Cc: Jann Horn Cc: Liu Shixin Cc: Muchun Song Signed-off-by: Andrew Morton (cherry picked from commit 14967a9c7d247841b0312c48dcf8cd29e55a4cc8) [ David: We don't have ptdesc and the wrappers, so work directly on the page->pt_share_count. CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING is still called CONFIG_ARCH_WANT_HUGE_PMD_SHARE. ] Signed-off-by: David Hildenbrand (Arm) --- mm/hugetlb.c | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index c0719ece2b7f..8268943e05eb 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4341,16 +4341,11 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, break; } - /* - * If the pagetables are shared don't copy or take references. - * - * dst_pte == src_pte is the common case of src/dest sharing. - * However, src could have 'unshared' and dst shares with - * another vma. So page_count of ptep page is checked instead - * to reliably determine whether pte is shared. - */ - if (page_count(virt_to_page(dst_pte)) > 1) +#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE + /* If the pagetables are shared, there is nothing to do */ + if (atomic_read(&virt_to_page(dst_pte)->pt_share_count)) continue; +#endif dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); -- 2.43.0