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Sun, 19 Apr 2026 15:00:08 -0400 X-MC-Unique: 19zZiZVLP4ql05oGpQ8RRw-1 X-Mimecast-MFC-AGG-ID: 19zZiZVLP4ql05oGpQ8RRw_1776625207 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AEB1218005A8; Sun, 19 Apr 2026 19:00:07 +0000 (UTC) Received: from p1.redhat.com (unknown [10.22.74.5]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 32837195608E; Sun, 19 Apr 2026 18:59:50 +0000 (UTC) From: Nico Pache To: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, npache@redhat.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: [PATCH 7.2 v16 05/13] mm/khugepaged: generalize collapse_huge_page for mTHP collapse Date: Sun, 19 Apr 2026 12:57:42 -0600 Message-ID: <20260419185750.260784-6-npache@redhat.com> In-Reply-To: <20260419185750.260784-1-npache@redhat.com> References: <20260419185750.260784-1-npache@redhat.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 X-Mimecast-MFC-PROC-ID: nQ02IXEkHECDgKlgV0Xnd19AGgV3-YQHCjPIgOlfQZ0_1776625207 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true Pass an order and offset to collapse_huge_page to support collapsing anon memory to arbitrary orders within a PMD. order indicates what mTHP size we are attempting to collapse to, and offset indicates were in the PMD to start the collapse attempt. For non-PMD collapse we must leave the anon VMA write locked until after we collapse the mTHP-- in the PMD case all the pages are isolated, but in the mTHP case this is not true, and we must keep the lock to prevent access/changes to the page tables. This can happen if the rmap walkers hit a pmd_none while the PMD entry is currently unavailable due to being temporarily removed during the collapse phase. Signed-off-by: Nico Pache --- mm/khugepaged.c | 103 +++++++++++++++++++++++++++--------------------- 1 file changed, 57 insertions(+), 46 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 283bb63854a5..ff6f9f1883ed 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1198,42 +1198,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru return SCAN_SUCCEED; } -static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address, - int referenced, int unmapped, struct collapse_control *cc) +static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr, + int referenced, int unmapped, struct collapse_control *cc, + unsigned int order) { LIST_HEAD(compound_pagelist); pmd_t *pmd, _pmd; - pte_t *pte; + pte_t *pte = NULL; pgtable_t pgtable; struct folio *folio; spinlock_t *pmd_ptl, *pte_ptl; enum scan_result result = SCAN_FAIL; struct vm_area_struct *vma; struct mmu_notifier_range range; + bool anon_vma_locked = false; + const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK; + const unsigned long end_addr = start_addr + (PAGE_SIZE << order); - VM_BUG_ON(address & ~HPAGE_PMD_MASK); - - /* - * Before allocating the hugepage, release the mmap_lock read lock. - * The allocation can take potentially a long time if it involves - * sync compaction, and we do not need to hold the mmap_lock during - * that. We will recheck the vma after taking it again in write mode. - */ - mmap_read_unlock(mm); - - result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER); + result = alloc_charge_folio(&folio, mm, cc, order); if (result != SCAN_SUCCEED) goto out_nolock; mmap_read_lock(mm); - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, - HPAGE_PMD_ORDER); + result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true, + &vma, cc, order); if (result != SCAN_SUCCEED) { mmap_read_unlock(mm); goto out_nolock; } - result = find_pmd_or_thp_or_none(mm, address, &pmd); + result = find_pmd_or_thp_or_none(mm, pmd_addr, &pmd); if (result != SCAN_SUCCEED) { mmap_read_unlock(mm); goto out_nolock; @@ -1245,8 +1239,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a * released when it fails. So we jump out_nolock directly in * that case. Continuing to collapse causes inconsistency. */ - result = __collapse_huge_page_swapin(mm, vma, address, pmd, - referenced, HPAGE_PMD_ORDER); + result = __collapse_huge_page_swapin(mm, vma, start_addr, pmd, + referenced, order); if (result != SCAN_SUCCEED) goto out_nolock; } @@ -1261,20 +1255,21 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a * mmap_lock. */ mmap_write_lock(mm); - result = hugepage_vma_revalidate(mm, address, true, &vma, cc, - HPAGE_PMD_ORDER); + result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true, + &vma, cc, order); if (result != SCAN_SUCCEED) goto out_up_write; /* check if the pmd is still valid */ vma_start_write(vma); - result = check_pmd_still_valid(mm, address, pmd); + result = check_pmd_still_valid(mm, pmd_addr, pmd); if (result != SCAN_SUCCEED) goto out_up_write; anon_vma_lock_write(vma->anon_vma); + anon_vma_locked = true; - mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address, - address + HPAGE_PMD_SIZE); + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, start_addr, + end_addr); mmu_notifier_invalidate_range_start(&range); pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */ @@ -1286,26 +1281,23 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a * Parallel GUP-fast is fine since GUP-fast will back off when * it detects PMD is changed. */ - _pmd = pmdp_collapse_flush(vma, address, pmd); + _pmd = pmdp_collapse_flush(vma, pmd_addr, pmd); spin_unlock(pmd_ptl); mmu_notifier_invalidate_range_end(&range); tlb_remove_table_sync_one(); - pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); + pte = pte_offset_map_lock(mm, &_pmd, start_addr, &pte_ptl); if (pte) { - result = __collapse_huge_page_isolate(vma, address, pte, cc, - HPAGE_PMD_ORDER, - &compound_pagelist); + result = __collapse_huge_page_isolate(vma, start_addr, pte, cc, + order, &compound_pagelist); spin_unlock(pte_ptl); } else { result = SCAN_NO_PTE_TABLE; } if (unlikely(result != SCAN_SUCCEED)) { - if (pte) - pte_unmap(pte); spin_lock(pmd_ptl); - BUG_ON(!pmd_none(*pmd)); + WARN_ON_ONCE(!pmd_none(*pmd)); /* * We can only use set_pmd_at when establishing * hugepmds and never for establishing regular pmds that @@ -1313,21 +1305,24 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a */ pmd_populate(mm, pmd, pmd_pgtable(_pmd)); spin_unlock(pmd_ptl); - anon_vma_unlock_write(vma->anon_vma); goto out_up_write; } /* - * All pages are isolated and locked so anon_vma rmap - * can't run anymore. + * For PMD collapse all pages are isolated and locked so anon_vma + * rmap can't run anymore. For mTHP collapse the PMD entry has been + * removed and not all pages are isolated and locked, so we must hold + * the lock to prevent neighboring folios from attempting to access + * this PMD until its reinstalled. */ - anon_vma_unlock_write(vma->anon_vma); + if (is_pmd_order(order)) { + anon_vma_unlock_write(vma->anon_vma); + anon_vma_locked = false; + } result = __collapse_huge_page_copy(pte, folio, pmd, _pmd, - vma, address, pte_ptl, - HPAGE_PMD_ORDER, - &compound_pagelist); - pte_unmap(pte); + vma, start_addr, pte_ptl, + order, &compound_pagelist); if (unlikely(result != SCAN_SUCCEED)) goto out_up_write; @@ -1337,18 +1332,27 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a * write. */ __folio_mark_uptodate(folio); - pgtable = pmd_pgtable(_pmd); - spin_lock(pmd_ptl); - BUG_ON(!pmd_none(*pmd)); - pgtable_trans_huge_deposit(mm, pmd, pgtable); - map_anon_folio_pmd_nopf(folio, pmd, vma, address); + WARN_ON_ONCE(!pmd_none(*pmd)); + if (is_pmd_order(order)) { /* PMD collapse */ + pgtable = pmd_pgtable(_pmd); + pgtable_trans_huge_deposit(mm, pmd, pgtable); + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); + } else { /* mTHP collapse */ + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false); + smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */ + pmd_populate(mm, pmd, pmd_pgtable(_pmd)); + } spin_unlock(pmd_ptl); folio = NULL; result = SCAN_SUCCEED; out_up_write: + if (anon_vma_locked) + anon_vma_unlock_write(vma->anon_vma); + if (pte) + pte_unmap(pte); mmap_write_unlock(mm); out_nolock: if (folio) @@ -1525,8 +1529,15 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, out_unmap: pte_unmap_unlock(pte, ptl); if (result == SCAN_SUCCEED) { + /* + * Before allocating the hugepage, release the mmap_lock read lock. + * The allocation can take potentially a long time if it involves + * sync compaction, and we do not need to hold the mmap_lock during + * that. We will recheck the vma after taking it again in write mode. + */ + mmap_read_unlock(mm); result = collapse_huge_page(mm, start_addr, referenced, - unmapped, cc); + unmapped, cc, HPAGE_PMD_ORDER); /* collapse_huge_page will return with the mmap_lock released */ *lock_dropped = true; } -- 2.53.0