From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id F1847CD5BD1 for ; Mon, 1 Jun 2026 03:28:59 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 47A766B020C; Sun, 31 May 2026 23:28:59 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 42B6B6B020D; Sun, 31 May 2026 23:28:59 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 27F116B020E; Sun, 31 May 2026 23:28:59 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 1080A6B020C for ; Sun, 31 May 2026 23:28:59 -0400 (EDT) Received: from smtpin26.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id B2AB91C1B10 for ; Mon, 1 Jun 2026 03:28:58 +0000 (UTC) X-FDA: 84829912356.26.21E8204 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) by imf07.hostedemail.com (Postfix) with ESMTP id EBAA24000C for ; Mon, 1 Jun 2026 03:28:56 +0000 (UTC) Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=pB5gPnos; spf=pass (imf07.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.185 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1780284537; a=rsa-sha256; cv=none; b=OsyKbAawTZ7eSnLohm0bJAH3jg8rDVaPZgN31juwcCUgN58bNIVIbwlDRW9kYLaqVyoVZv 89ULlRUGEp1aP15eJ2SxrzxoMR+sZjlleT48Li2kxZjL+f+WE8A5VnSeKA0VF4Q9qhzVIv RPwTu6/NmolJEss+HDnEU0TOfbwOOgU= ARC-Authentication-Results: i=1; imf07.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=pB5gPnos; spf=pass (imf07.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.185 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1780284537; h=from:from:sender: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:dkim-signature; bh=JGqe1xP59WaPQHJPwgOhJ6HEmEBn7hlp4YbZ8TjEsWg=; b=H2jp84HKHGQfGaEyeLZ7bR71kD6Gh4rLWAhacrtmH/tycd4k1eZU+10Al7cQn4YHjB34ue 3JqVWqid425r4C9P2rtyPJVLFnDH0CJtvoEMsdAkyh1AXgnZdnyrKV+xgXnT3THyj5VVxA HZ4hOqK5a+fdU8eSkLkzyuES3mQCRf4= 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> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: EBAA24000C X-Stat-Signature: uy1iornjjcjg3k4iu1mmbm3jjxwsknui X-HE-Tag: 1780284536-975838 X-HE-Meta: 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 orfxgyPT HqsKD4E1exQyRp8wy6Eid0tru5kcTj3UIdbJK7AqN1eUguKJhlNfmyDoyj+FK/Xx5ddKaaQArWodISasWP6xQ6TOBZgaXGDwgJ4fGjeqSGafmwWGLcvhSwZVkjSKw8dMFsVXhSmTXQ5GCfeRucm8gSlr1BlDuFKJ0bQt+QlwUWypXXhGrh5gOTqGm4W2H6kskRjvaypbFYYJZfNzzT1W4hPYwAy4HUtx/jNKw1YMeTRq92oYOwLba6tU+kusKZpx5DfvSLg3ElVOuLkLv7ADISlLWrRjv3swGF44kSiVy16uGDBzddGeypW6DhBBUeq5eTzrhfWCBsnXFDAp8r25j+W/X6G4VEqcaGP8jVNl1mIKX5jaL0mOpm/6/UrmeUgv2esIS Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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