From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793306276; b=W+aKM6HddZc75Iznc5ztsOhBjx6fVIw/Y50Iesf+kGwNph495s8c31G5FM+wpZPAhtJ5 cZs4N3l8vycBBbFQvxpDOwKVgYvpO1/ftjXMWnNvlluWAg9s5WD4fQ9S1H72J1YBLVTM2 xCGLvMuOOHZ+PnhcbrxjP+GDn6wqkoQLuDVUg32r/7/k9Y/laIPuP4wU3kJbPZpXwCA7q oHGyjBcNh1p2C7ulBy0E6OirXKDuYwIpdw6u6j+Sv8UqhNyoG0CbEcL8ymsdP26nuQvW1 i0Y0HMFR/DEpoCCz9+/9lMobFxctzQJaTZ8H+idUMNEku0RdJm3HHA9zWD/hH3ffLDg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793306276; h=Received:DKIM-Signature:From:Date:Subject:MIME-Version:Content-Type: Content-Transfer-Encoding:Message-Id:References:In-Reply-To:To:Cc: X-Mailer:X-Developer-Signature:X-Developer-Key; bh=tztEcWM4A0N7b+6teiPYZ27u+mDO7RxR1zwT2bOzVVo=; b=h0KcNL7BgJ31/4ZKh60xNddxOhi9IkaRuHXAxcSjDLmLJspbkxrxj4Rit9e6Hp2Po23m 5Z37XU79sObPm3HJRUtDQryiOpZw62XW9lH+OB9GZXGGecTwAfOrhqWQpDSnKzS3mj+z+ 2wfeQJHc0tKKjz7Zg66nCtRzc7WHyFM1VZGuJe/5+NpP9x0erxZHKkNHmvroMo8xSPW86 BsGdlAs5HlZ+nDV9oBQzhunWgQFVuPTPNVbiB/IiQ0yAM00HiZ40Un8DfWsO6NKg4NiDL YCf0Q11/KWXS8Oavpmd3IUiY42jH19S5N3ubW6mO+W9dAARW56aQu2Wa/vXUINnLLwQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=o2acdNnV; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DE010606A31C; Thu, 29 Oct 2026 20:37:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=o2acdNnV Received: by smtp.kernel.org (Postfix) with ESMTPSA id A358F1F01816; Thu, 29 Oct 2026 20:37:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793306275; bh=tztEcWM4A0N7b+6teiPYZ27u+mDO7RxR1zwT2bOzVVo=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=o2acdNnV4CkuFKbRpEwNEOnOSL9pTT5uHY41RgahtBUOZ7qn8kMmJu1nKIihtbJfl GEjozaA3IsN8l1U69dN2g6SsJAMhpcDzsMmD+nQB7WGSqybBiSHBKE005LMdfadMXD 4qAQYfy8Zj/9BXND3KpgaU5qwcaxk+xW7+nuT+B9gVnZpt/ddcdAquZtG3aQaa+Ya/ txypaW4mgwQTIX4XYZJzdsRPgQJEL5Sy3NYTNsfIBeIUA9dtjU1zhIS4E4d186vBWt JxAe9i3rjiOBhKdz9CYsah0lxU97FDpI/vbqVULgw1QyIFTGx9Gfz1FxZAqmo1wTHB CaoSm3PuAzuBg== From: "David Hildenbrand (Arm)" Date: Thu, 29 Oct 2026 21:33:13 +0100 Subject: [PATCH v2 18/47] mm/khugepaged: check if PTE table is empty in try_collapse_pte_mapped_thp() X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261029-fixes_core-v2-18-a64ce7f83abb@kernel.org> References: <20261029-fixes_core-v2-0-a64ce7f83abb@kernel.org> In-Reply-To: <20261029-fixes_core-v2-0-a64ce7f83abb@kernel.org> To: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, "David Hildenbrand (Arm)" X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5636; i=david@kernel.org; h=from:subject:message-id; bh=8iTA5/vUtqz9osOYx6RyePWtnzJPnHa6IV6FgiGHX/4=; b=owGbwMvMwCF2/OVpCdfQ3ZWMp9WSGLIer20r1Knvm/L+wvJF2cwfNQ1TTWy8j7GUzk/p+BGzr MAtdv36jlIWBjEOBlkxRZag4PsGL1+uq61242OGmcPKBDKEgYtTACZSv5fhn9HEh0pTZ83dL7hF R5Pp5FSha793iwT1xrY0RdxP0nj7/SrDP10F5TMvHH9cEz/WpeiX/1glmpElL0DWv+5iyMVQh9x 3PAA= X-Developer-Key: i=david@kernel.org; a=openpgp; fpr=1BD9CAAD735C4C3A460DFCCA4DDE10F700FF835A The existing code is rather horrible and already resulted in bugs in the past: we special-case UFFD inserting anonymous folios by grabbing the PMD lock early, but ignore any other markers (guard, poison through uffd) that could have been inserted into the page table in the meantime, effectively dropping them. Let's just unify and simplify the locking: try to grab the PMD table lock and if that fails, flush the TLB, drop the PTE lock, take PT locks in the right order and check if the page table is still empty. We still perform the heavy lifting (zapping the folio ptes) under PTE table lock only. Include for now, as x86's flush_tlb_range() macro depends on hugetlb helpers. This dependency needs a separate cleanup. Both issues this fixes are rather corner cases. Let's not CC stable for now. Fixes: f807123d578d ("mm: allow guard regions in file-backed and read-only mappings") Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()") Tested-by: Gregory Price (Meta) Reviewed-by: Gregory Price (Meta) Signed-off-by: David Hildenbrand (Arm) --- mm/khugepaged.c | 64 ++++++++++++++++++++++++++++----------------------------- 1 file changed, 31 insertions(+), 33 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index dd17c3dae800..4313abf9cfcb 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -10,6 +10,7 @@ #include #include #include +#include #include #include #include @@ -1832,6 +1833,17 @@ static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long a return SCAN_SUCCEED; } +static bool pte_table_empty(pte_t *pte) +{ + int i; + + for (i = 0; i < HPAGE_PMD_NR; i++, pte++) { + if (!pte_none(ptep_get(pte))) + return false; + } + return true; +} + static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, bool install_pmd) { @@ -1935,33 +1947,14 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign } pte_unmap_unlock(start_pte, ptl); - ptl = NULL; mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, haddr, haddr + HPAGE_PMD_SIZE); mmu_notifier_invalidate_range_start(&range); notified = true; - /* - * pmd_lock covers a wider range than ptl, and (if split from mm's - * page_table_lock) ptl nests inside pml. The less time we hold pml, - * the better; but userfaultfd's mfill_atomic_pte() on a private VMA - * inserts a valid as-if-COWed PTE without even looking up page cache. - * So page lock of folio does not protect from it, so we must not drop - * ptl before pgt_pmd is removed, so uffd private needs pml taken now. - */ - if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED)) - pml = pmd_lock(mm, pmd); - - start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl); + start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); if (!start_pte) goto abort; - if (!pml) - spin_lock(ptl); - else if (ptl != pml) - spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); - - if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) - goto abort; /* step 2: clear page table and adjust rmap */ for (i = 0, addr = haddr, pte = start_pte; i < HPAGE_PMD_NR; @@ -1992,10 +1985,7 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign nr_batch_ptes = folio_pte_batch(folio, pte, ptent, max_nr_batch_ptes); - /* - * Must clear entry, or a racing truncate may re-remove it. - * TLB flush can be left until pmdp_collapse_flush() does it. - */ + /* Must clear entry, or a racing truncate may re-remove it. */ ptent = get_and_clear_ptes(mm, addr, pte, nr_batch_ptes); if (pte_dirty(ptent)) folio_mark_dirty(folio); @@ -2003,25 +1993,33 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign nr_mapped_ptes += nr_batch_ptes; } - if (!pml) - spin_unlock(ptl); - /* step 3: set proper refcount and mm_counters. */ if (nr_mapped_ptes) { folio_ref_sub(folio, nr_mapped_ptes); add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes); } - /* step 4: remove empty page table */ - if (!pml) { + /* step 4: acquire the PMD lock and remove the empty page table */ + pml = pmd_lockptr(mm, pmd); + if (ptl != pml && !spin_trylock(pml)) { + /* We must flush before dropping the PTL. */ + if (nr_mapped_ptes) + flush_tlb_range(vma, haddr, end); + pte_unmap_unlock(start_pte, ptl); + ptl = NULL; + pml = pmd_lock(mm, pmd); + start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl); + if (!start_pte) + goto unlock; if (ptl != pml) spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); - if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) { - flush_tlb_mm(mm); + + if (!pte_table_empty(start_pte)) goto unlock; - } } + + /* This also flushes the zapped folio ptes properly. */ pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd); pmdp_get_lockless_sync(); pte_unmap_unlock(start_pte, ptl); @@ -2042,7 +2040,7 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign goto drop_folio; abort: if (nr_mapped_ptes) { - flush_tlb_mm(mm); + flush_tlb_range(vma, haddr, end); folio_ref_sub(folio, nr_mapped_ptes); add_mm_counter(mm, mm_counter_file(folio), -nr_mapped_ptes); } -- 2.43.0