From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A200C19CC14 for ; Mon, 1 Jun 2026 03:28:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780284539; cv=none; b=SWs9RRI1NU7qNjFHZS7HkYZJQbtm4AxB8zL7Wcpqi5JX6WlwMiMX+dPMBTQ9uryUt39ppr7hnG2lyX4T3h1++E+Xw9f2T0CxxtAtEppGnw5IU6TbE2j7A8hIBaaKILaqAgMhBbBCIRnqQjtRMXlF73lG+NDUmlEAdOt7hyj2bvE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780284539; c=relaxed/simple; bh=xXb7WP3F0K39sxQbdFXi4iNvPZUb8tntO5MzvQy3Olo=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=eBfNcEwBlgM7yWlkOV6N7Wy1Krr9XIZSGHgupHRPUtAEcoF0zXiYLotLJOt2CMqh/+FrAcy14lU1wEp5iKNNB5vsTEycVSTYFCm7VAXe4nOws7GfpKz8dW58KzggYLjZIXtR0f21bQNCj4A+hk10BdnkOVeHT1laPlt3AdCV0wI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=pB5gPnos; arc=none smtp.client-ip=91.218.175.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="pB5gPnos" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780284534; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=JGqe1xP59WaPQHJPwgOhJ6HEmEBn7hlp4YbZ8TjEsWg=; b=pB5gPnosZrseYYPqwmoGXuQMwKGKOs4C0M9gSF0jM0TC+3sb11mw1MAL/OqH2PFg3bKiGg DIu87NJ8sRdc7plHLulyU7UR1XzwEkbjUmiKa25DEwUgng7xT/7HBYISi3dO01QyYPpyEF Y0RzaG2mv/G6ZJW4ahIt40m/ty9oE84= From: Lance Yang To: david@kernel.org Cc: lance.yang@linux.dev, npache@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, 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, 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, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.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, usama.arif@linux.dev Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse Date: Mon, 1 Jun 2026 11:28:04 +0800 Message-Id: <20260601032804.96122-1-lance.yang@linux.dev> In-Reply-To: <2024af56-5e99-4799-a586-e9ba756cecb9@kernel.org> References: <2024af56-5e99-4799-a586-e9ba756cecb9@kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Sun, May 31, 2026 at 10:00:17PM +0200, David Hildenbrand (Arm) wrote: >On 5/31/26 11:39, Lance Yang wrote: >> >> On Fri, May 22, 2026 at 09:00:01AM -0600, Nico Pache wrote: >>> 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. >>> >>> Acked-by: Usama Arif >>> Signed-off-by: Nico Pache >>> --- >>> mm/khugepaged.c | 93 +++++++++++++++++++++++++++++-------------------- >>> 1 file changed, 55 insertions(+), 38 deletions(-) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index fab35d318641..d64f42f66236 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1214,34 +1214,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru >>> * while allocating a THP, as that could trigger direct reclaim/compaction. >>> * Note that the VMA must be rechecked after grabbing the mmap_lock again. >>> */ >>> -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) >>> { >>> + const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK; >>> + const unsigned long end_addr = start_addr + (PAGE_SIZE << 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; >>> >>> - VM_BUG_ON(address & ~HPAGE_PMD_MASK); >>> - >>> - 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; >>> @@ -1253,8 +1255,8 @@ 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; >>> } >>> @@ -1269,20 +1271,21 @@ 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 */ >>> @@ -1294,26 +1297,23 @@ 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 >>> @@ -1321,21 +1321,24 @@ 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; >>> >>> @@ -1345,18 +1348,32 @@ 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)) { >>> + pgtable = pmd_pgtable(_pmd); >>> + pgtable_trans_huge_deposit(mm, pmd, pgtable); >>> + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); >>> + } else { >>> + /* >>> + * set_ptes is called in map_anon_folio_pte_nopf with the >>> + * pmd_ptl lock still held; this is safe as the PMD is expected >>> + * to be none. The pmd entry is then repopulated below. >>> + */ >>> + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false); >> >> Emm ... is it safe to use map_anon_folio_pte_nopf() here? >> >> At this point pmdp_collapse_flush() has cleared the PMD from the page >> tables. The PTE table we are updating is only reachable through the saved >> old PMD value, _pmd, until pmd_populate() below. >> >> map_anon_folio_pte_nopf() does set_ptes() and then calls >> update_mmu_cache_range(). Documentation/core-api/cachetlb.rst describes >> that hook as: >> >> " >> At the end of every page fault, this routine is invoked to tell >> the architecture specific code that translations now exists >> in the software page tables for address space "vma->vm_mm" >> at virtual address "address" for "nr" consecutive pages. >> " >> >> But that does not seem true here yet, since the PTE table is not >> reachable from vma->vm_mm when update_mmu_cache_range() is called. >> >> Should we avoid calling update_mmu_cache_range() until after the PTE >> table is reinstalled with pmd_populate()? > >I recall that update_mmu_cache* users mostly care about updating folios flags, >for the folio derived from the PTE ... or flushing caches for the user address. > >So intuitively I would say "the architecture code doesn't care that the PMD >table will only be visible to HW shortly after". The important thing should be >that it will definetly happen, and that nothing else is curently there or can be >there? Ah, fair point. I was mostly worried about arch hooks that walk vma->vm_mm again, rather than only using the pte pointer passed in. For example, mips does: update_mmu_cache_range() -> __update_tlb() -> pgd_offset(vma->vm_mm, address) -> pte_offset_map(...) and __update_tlb() has this assumption: /* * update_mmu_cache() is called between pte_offset_map_lock() * and pte_unmap_unlock(), so we can assume that ptep is not * NULL here: and what should be done below if it were NULL? */ So if khugepaged happens to run with current->active_mm == vma->vm_mm here, could __update_tlb() hit the none PMD, get NULL from pte_offset_map(), and then dereference it? Just wanted to raise it since some arch code may still have assumptions like this, and the always-enable-mTHP work is getting closer ... Probably very very very hard to hit, though :) Cheers, Lance