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Sun, 26 Apr 2026 02:19:58 -0700 (PDT) Date: Sun, 26 Apr 2026 09:19:57 +0000 From: Wei Yang To: "David Hildenbrand (Arm)" Cc: Wei Yang , akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, riel@surriel.com, vbabka@kernel.org, harry@kernel.org, jannh@google.com, rppt@kernel.org, surenb@google.com, mhocko@suse.com, shuah@kernel.org, linux-mm@kvack.org, Gavin Guo Subject: Re: [PATCH 1/2] mm/huge_memory: return true if split_huge_pmd_locked() split PMD to migration entry Message-ID: <20260426091957.a227zxgkqapibtud@master> Reply-To: Wei Yang References: <20260415010839.20124-1-richard.weiyang@gmail.com> <20260415010839.20124-2-richard.weiyang@gmail.com> <79e164a2-47ce-4a02-82f5-164515760b6d@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <79e164a2-47ce-4a02-82f5-164515760b6d@kernel.org> User-Agent: NeoMutt/20170113 (1.7.2) X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 65CD61A0006 X-Stat-Signature: j37t7kredi847waauq5tkahw1ckkk7rh X-Rspam-User: X-HE-Tag: 1777195201-789294 X-HE-Meta: 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 TZp3tI0h 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 PcSNCFpv /Et8HFCwz2zM/0djS5yZSQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 24, 2026 at 09:29:18PM +0200, David Hildenbrand (Arm) wrote: >On 4/15/26 03:08, Wei Yang wrote: >> When @freeze is set to true, split_huge_pmd_locked() is intended to >> split the PMD to migration entry. But if it doesn't manage to clear >> PageAnonExclusive(), it just split PMD and leave the folio mapped >> through PTE. >> >> This patch let split_huge_pmd_locked() return true to indicate it does >> split PMD to migration entry. With this knowledge, we can return >> directly in try_to_migrate_one() if it does. >> >> Signed-off-by: Wei Yang >> Cc: Gavin Guo >> Cc: "David Hildenbrand (Red Hat)" >> Cc: Zi Yan >> Cc: Baolin Wang >> Cc: Lance Yang >> --- > >[...] > >> static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma, >> unsigned long addr, pmd_t *pmdp, >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 970e077019b7..ec84bb4a0cc3 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3087,7 +3087,7 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma, >> pmd_populate(mm, pmd, pgtable); >> } >> >> -static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> +static bool __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> unsigned long haddr, bool freeze) >> { >> struct mm_struct *mm = vma->vm_mm; >> @@ -3096,7 +3096,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> pgtable_t pgtable; >> pmd_t old_pmd, _pmd; >> bool soft_dirty, uffd_wp = false, young = false, write = false; >> - bool anon_exclusive = false, dirty = false; >> + bool anon_exclusive = false, dirty = false, ret = false; >> unsigned long addr; >> pte_t *pte; >> int i; >> @@ -3118,13 +3118,13 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> if (arch_needs_pgtable_deposit()) >> zap_deposited_table(mm, pmd); >> if (vma_is_special_huge(vma)) >> - return; >> + return ret; > >Why not "return false" in these cases where it really can always only false? > Will adjust related places. >> if (unlikely(pmd_is_migration_entry(old_pmd))) { >> const softleaf_t old_entry = softleaf_from_pmd(old_pmd); >> >> folio = softleaf_to_folio(old_entry); >> } else if (is_huge_zero_pmd(old_pmd)) { >> - return; >> + return ret; >> } else { >> page = pmd_page(old_pmd); >> folio = page_folio(page); >> @@ -3136,7 +3136,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> folio_put(folio); >> } >> add_mm_counter(mm, mm_counter_file(folio), -HPAGE_PMD_NR); >> - return; >> + return ret; >> } >> >> if (is_huge_zero_pmd(*pmd)) { >> @@ -3149,7 +3149,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> * small page also write protected so it does not seems useful >> * to invalidate secondary mmu at this time. >> */ >> - return __split_huge_zero_page_pmd(vma, haddr, pmd); >> + __split_huge_zero_page_pmd(vma, haddr, pmd); >> + return ret; >> } >> >> if (pmd_is_migration_entry(*pmd)) { >> @@ -3309,6 +3310,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> VM_WARN_ON(!pte_none(ptep_get(pte + i))); >> set_pte_at(mm, addr, pte + i, entry); >> } >> + ret = true; >> } else if (pmd_is_device_private_entry(old_pmd)) { >> pte_t entry; >> swp_entry_t swp_entry; >> @@ -3366,14 +3368,17 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> >> smp_wmb(); /* make pte visible before pmd */ >> pmd_populate(mm, pmd, pgtable); >> + return ret; >> } >> >> -void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, >> +bool split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, >> pmd_t *pmd, bool freeze) >> { >> VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE)); >> if (pmd_trans_huge(*pmd) || pmd_is_valid_softleaf(*pmd)) >> - __split_huge_pmd_locked(vma, pmd, address, freeze); >> + return __split_huge_pmd_locked(vma, pmd, address, freeze); >> + else >> + return false; > >No need for the "else". > Got it. >> } >> >> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 78b7fb5f367c..91fb495bebbe 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2464,13 +2464,18 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, >> >> if (flags & TTU_SPLIT_HUGE_PMD) { >> /* >> - * split_huge_pmd_locked() might leave the >> + * If split_huge_pmd_locked() does split PMD >> + * to migration entry, we are done. >> + * If split_huge_pmd_locked() leave the >> * folio mapped through PTEs. Retry the walk >> * so we can detect this scenario and properly >> * abort the walk. > >Couldn't we just abort right away, based on the return value? > Here is my understanding. We get here when page_vma_mapped_walk() touch a pmd entry, with three cases: * pmd_trans_huge() * pmd_is_migration_entry() * pmd_is_device_private_entry() For the first two cases, we grab pmd_lock() and then check the condition is still valid before return. But for case 3, after grab pmd_lock(), it return directly. This may give chance for another thread to split pmd_is_device_private_entry() to pte mapped, IIUC. For this case, we should restart the walk here. Not sure I got it correctly. -- Wei Yang Help you, Help me