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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1780220398; 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=CDqpOm4GUMbmXnNlKMQcoCkHdGOfT4SYZ50geLI7x88=; b=cBPRo2xpHgEFrS06avLHNIBZGggrrVtlB5XZ+WsYxzDe2PXPZRngmY+/U1AUH7pR5wkT2Q vnplr+blS3IBIuHIWL4br9MlrOAlw02LleDz+ZfMpxxHTb4BA8VqAyZsPGwW+psK0U3mb2 to1DOY5Cw2X6iSU8cz4Otw+jhAK5KGg= From: Lance Yang To: npache@redhat.com 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, 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: Sun, 31 May 2026 17:39:42 +0800 Message-Id: <20260531093942.19644-1-lance.yang@linux.dev> In-Reply-To: <20260522150009.121603-7-npache@redhat.com> References: <20260522150009.121603-7-npache@redhat.com> 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: DCC992000D X-Stat-Signature: 1sn8xcscc9edi5oke4s11yo3he36aabp X-HE-Tag: 1780220400-78973 X-HE-Meta: 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 8qQD/cbd iq8jwu8+un7unz8x5q1xHsJr8kNTXW59rEgtbsovVK1cm1Uxlq9vggw5W7+theLJ5kl6aOFDo8+diqSDPOx0EFUs3eTFDXArmvEo/FtKPDcr2xD0TwlXxdiqv89EbF2E2hLHMn8i7BDjPlc1HhViGk2KFgs+lOWI1y99brKk6fwksN3eiSWnl2UlbUwsKUB6XKgfzcgYyb3tx0it/YXGkow+GwVSh1vyFXTsfMUGusVdcJtCGl/NFQBQkunaO3GCR0BDh2TOt6/Qpe8Ngk9pWRlXHaHOWZMOVJgysJDuCRZzxoKWRaNXYf5D/Npd1UwAzwNtnHr/ULb9a+fLLFMjSryIQIggje+RrZ9Isdna0fPSBsDQRG5lOBQbK8TLxZDwT/vCA Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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()? Cheers, Lance >+ 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) >@@ -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 > >