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b=mFxoq+6dwmB63YA4eNje+poauaE108YtMyQzjeUIVWivqr6B0Z/N/8uE7lnIo9Nd4 FFIU1+jAIvqnVvjFOwuRuahuM8yP+S33zThST2xFhJ83iYtjfYWvQ5xC+x3ClIQASI IECYBxbT5ufcfQ7SbE6M+9B24G0iFU9XRjq94SKuCszF17U5xrVRsddXrIQafQEzlj M34f2pOG/jPs2YDq9Its1aG37nFh5cf91N4L3mmDMpRCOXk9EKD/x8rwC9kOt3qVVw YBg8fusqS3krzm/TSgeqRd+CAlp7r8NGhUU4/HLkYJ2KTvUhgZhmCVdfTB6jSnE7XJ 3H2uIvr2GPzfg== Date: Thu, 4 Jun 2026 12:38:16 +0100 From: Lorenzo Stoakes To: Nico Pache Cc: 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, 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@infradead.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 Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse Message-ID: References: <20260522150009.121603-1-npache@redhat.com> <20260522150009.121603-7-npache@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260522150009.121603-7-npache@redhat.com> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A600CA0005 X-Stat-Signature: njwperie7p8xgrqdgak5au139hrepdbi X-Rspam-User: X-HE-Tag: 1780573111-307745 X-HE-Meta: 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 ANOswazf iv9pGBqdBFjCJfF3OLtc0nZ1p4DaIM9EzyeFn4ZyTLydQ5YlOkakqSjME0DCgAViazlxAyh0bq6TnL7cPv9WjaGeOJOAznELMAa15MM53LbbeJJl7gU8TqlaTH0NZWOhnqS1mm2ZvKM4eb3aU6i2A79trmRevTcSbpNzgn1Wf7Mj6DV9f/ci5hWPxRke1rpSD91eVdxZ+D1ZXoQMcfB2yZ2Y13EOq/ozj2zGcM3K4hYc2VeE1++4lLGY+jXZv/L/JpMHRQCQjocrE3883C+qD98mcN6PmrUauugBOeVL/2wDfSPKkfFzntjJ9nuOW5/LhcZc1xIES0Fep9IDrTySkdSZLGKcDaEIABwdzZ19roK7uLzGzk3aVedJYKrEsbRwoJAOMeJeXUSZSwfY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: I will go review the thread about the cache maintenance separately and respond about that. 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 The logic LGTM generally, some questions for understanding below, and of course as per above I want to review the Lance/David subthread. Thanks! > --- > 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; As mentioned elsewhere for some reason this was dropped in mm-unstable. Maybe a bad conflict resolution? > 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; I worry that we hold this lock a lot longer now? Maybe the algorithmic change alters that, but Claude did suggest on the s390 bug that longer lock hold might be an issue. I wonder if we'll observe lock contention as a result? Correct me if I'm wrong and we're not holding longer than previously, however. Just appears that we do. > > - 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); OK I seem to remember this is because we're holding the anon_vma lock longer. That does imply that on e.g. x86-64 the RCU lock is being held a bit longer also as well as the anon_vma loc. I guess it's also because we need to hold anon_vma and pte lock because we're fiddling around at PTE level for mTHP not just PMD level as 'classic' THP did. (Rememberings going on here :) > 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 Right because some PTE entries be unaffected by the change. > + * the lock to prevent neighboring folios from attempting to access > + * this PMD until its reinstalled. OK. This is slightly annoying for my CoW context work as it means there's another case where we need to explicitly hold an anon_vma lock for correctness :) Anyway I will think about that separately, is what it is. And in fact motivates to want this merged earlier so I can work against it :) > */ > - 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 PMD entry you mean? > + * to be none. The pmd entry is then repopulated below. > + */ > + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false); So here we populate entries in the existing PTE _table_ to point at the new order>0 folio? With arm64 of course doing transparent contpte stuff? > + smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */ > + pmd_populate(mm, pmd, pmd_pgtable(_pmd)); And then we reinstall the pre-existing PMD _entry_ from none -> what it was before? > + } > 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) > @@ -1536,7 +1553,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, > /* collapse_huge_page expects the lock to be dropped before calling */ > 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; > } > -- > 2.54.0 >