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b=LJzLebHvw46Bwf+3ndXp1UQobNZ8HBCK+xk7JuTqBoFdQiF38q+b/cAP/YYQv4pgy ICmtHt6f+n1JxdKleWo+hsiDruv+iEJrKFq8b3SjhXdMRhM1gOZwB2y89ijqJfBbzw xRmBrEuLA5zO7IhUj/UrShkrcX3yZ0PCJNh8nOkUPwm4VLl4bVGoEPNmA8jKZ+G1Xk NDgafYsgI/7HCYz8+62oILimCf1NiHUzDd9pX1pajG9+jjItBiDLiKsLupc5X1hU3e BKGpeDHgxrxzZkN9AuH6BFMlZJc0ad9r9Cdl+xGOPmzvrBMKbLFOZjsWTZUS9Yw6pZ 92+mEjHGhdbow== 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 6.1.y 1/4] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count Date: Wed, 18 Feb 2026 10:16:04 +0100 Message-ID: <20260218091608.25726-2-david@kernel.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260218091608.25726-1-david@kernel.org> References: <2026012605-uncorrupt-yanking-4155@gregkh> <20260218091608.25726-1-david@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: j4xxk8t1mnos5j35z7af9tpnddj9jrnh X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B3D3340003 X-HE-Tag: 1771406192-734657 X-HE-Meta: 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 mB47dMGX lZzhN+4zX3eThxVH55oviGwa0jy4xTRQuVM/PVrxYNASsDW8WFQoN2yIwu525VS4NRL4764hcs5NFFgmuaffTWlQaS1+E34MgA29B4+A3zDyR914jMfAAK/zRd3GjYuys3Jeih2KqJ/yAgIuzloMCknd9otKkT6Wt+S86DNotV54/N4jxuNUehmIaPRwImGEFn/uNAZGZTsQMVaVcCY+BapXcHeyDFfanJyVMfJpEOGQaiH1prNvq2xp2VFaghN+BZ414HzQQHjZjVvWkMq3zz3WL371y/h6oW7Z34M7r60wB8FSsS3huyZvfI8W6sPcUDVZC7/OLXoMMm1tSDcJYvZ6O4vqdwKJGOI9Jh89qHbhlz7iPa041NCvSbfg03y84bg2Z6P87q3r6LXgILTa+j2yULuZPQ17O3Yc2PBSGGwpyiiIKmzgbIAs/Xw== 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 e7bac08071de..b07b332beabb 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5084,18 +5084,13 @@ 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)) { addr |= last_addr_mask; continue; } +#endif dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); -- 2.43.0