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[2003:cb:c703:fe00:10fe:298:6bf1:d163]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-422874e73bcsm52580255e9.41.2024.06.13.02.25.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 13 Jun 2024 02:25:16 -0700 (PDT) Message-ID: <61dbc0f7-632f-4179-8c59-616c9cfabaf1@redhat.com> Date: Thu, 13 Jun 2024 11:25:15 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 3/4] mm/rmap: integrate PMD-mapped folio splitting into pagewalk loop To: Lance Yang Cc: 21cnbao@gmail.com, akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, fengwei.yin@intel.com, libang.li@antgroup.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, maskray@google.com, mhocko@suse.com, minchan@kernel.org, peterx@redhat.com, ryan.roberts@arm.com, shy828301@gmail.com, sj@kernel.org, songmuchun@bytedance.com, wangkefeng.wang@huawei.com, willy@infradead.org, xiehuan09@gmail.com, ziy@nvidia.com, zokeefe@google.com References: <20240610120209.66311-1-ioworker0@gmail.com> <20240610120618.66520-1-ioworker0@gmail.com> <933c7339-2dbd-464b-b342-e4cff7ad75a3@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 13.06.24 11:21, Lance Yang wrote: > On Thu, Jun 13, 2024 at 4:34 PM David Hildenbrand wrote: >> >> On 10.06.24 14:06, Lance Yang wrote: >>> In preparation for supporting try_to_unmap_one() to unmap PMD-mapped >>> folios, start the pagewalk first, then call split_huge_pmd_address() to >>> split the folio. >>> >>> Suggested-by: David Hildenbrand >>> Suggested-by: Baolin Wang >>> Signed-off-by: Lance Yang >>> --- >>> include/linux/huge_mm.h | 6 ++++++ >>> mm/huge_memory.c | 42 +++++++++++++++++++++-------------------- >>> mm/rmap.c | 21 +++++++++++++++------ >>> 3 files changed, 43 insertions(+), 26 deletions(-) >>> >>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h >>> index 088d66a54643..4670c6ee118b 100644 >>> --- a/include/linux/huge_mm.h >>> +++ b/include/linux/huge_mm.h >>> @@ -415,6 +415,9 @@ static inline bool thp_migration_supported(void) >>> return IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION); >>> } >>> >>> +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, >>> + pmd_t *pmd, bool freeze, struct folio *folio); >>> + >>> #else /* CONFIG_TRANSPARENT_HUGEPAGE */ >>> >>> static inline bool folio_test_pmd_mappable(struct folio *folio) >>> @@ -477,6 +480,9 @@ static inline void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >>> unsigned long address, bool freeze, struct folio *folio) {} >>> static inline void split_huge_pmd_address(struct vm_area_struct *vma, >>> unsigned long address, bool freeze, struct folio *folio) {} >>> +static inline void split_huge_pmd_locked(struct vm_area_struct *vma, >>> + unsigned long address, pmd_t *pmd, >>> + bool freeze, struct folio *folio) {} >>> >>> #define split_huge_pud(__vma, __pmd, __address) \ >>> do { } while (0) >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index e6d26c2eb670..d2697cc8f9d4 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -2581,6 +2581,27 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >>> pmd_populate(mm, pmd, pgtable); >>> } >>> >>> +void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, >>> + pmd_t *pmd, bool freeze, struct folio *folio) >>> +{ >>> + VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio)); >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE)); >>> + VM_WARN_ON_ONCE(folio && !folio_test_locked(folio)); >>> + VM_BUG_ON(freeze && !folio); >> >> Curious: could we actually end up here without a folio right now? That >> would mean, that try_to_unmap_one() would be called with folio==NULL. > > try_to_unmap_one() would not be called with folio==NULL, I guess. > > I just moved 'VM_BUG_ON(freeze && !folio)' from __split_huge_pmd() to here, > and now __split_huge_pmd() will call split_huge_pmd_locked(). Right, on that path we could likely get !folio. > >> >>> + >>> + /* >>> + * When the caller requests to set up a migration entry, we >>> + * require a folio to check the PMD against. Otherwise, there >>> + * is a risk of replacing the wrong folio. >>> + */ >>> + if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || >>> + is_pmd_migration_entry(*pmd)) { >>> + if (folio && folio != pmd_folio(*pmd)) >>> + return; >>> + __split_huge_pmd_locked(vma, pmd, address, freeze); >>> + } >>> +} >>> + >>> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >>> unsigned long address, bool freeze, struct folio *folio) >>> { >>> @@ -2592,26 +2613,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, >>> (address & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE); >>> mmu_notifier_invalidate_range_start(&range); >>> ptl = pmd_lock(vma->vm_mm, pmd); >>> - >>> - /* >>> - * If caller asks to setup a migration entry, we need a folio to check >>> - * pmd against. Otherwise we can end up replacing wrong folio. >>> - */ >>> - VM_BUG_ON(freeze && !folio); >>> - VM_WARN_ON_ONCE(folio && !folio_test_locked(folio)); >>> - >>> - if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || >>> - is_pmd_migration_entry(*pmd)) { >>> - /* >>> - * It's safe to call pmd_page when folio is set because it's >>> - * guaranteed that pmd is present. >>> - */ >>> - if (folio && folio != pmd_folio(*pmd)) >>> - goto out; >>> - __split_huge_pmd_locked(vma, pmd, range.start, freeze); >>> - } >>> - >>> -out: >>> + split_huge_pmd_locked(vma, range.start, pmd, freeze, folio); >>> spin_unlock(ptl); >>> mmu_notifier_invalidate_range_end(&range); >>> } >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index ddffa30c79fb..b77f88695588 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1640,9 +1640,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>> if (flags & TTU_SYNC) >>> pvmw.flags = PVMW_SYNC; >>> >>> - if (flags & TTU_SPLIT_HUGE_PMD) >>> - split_huge_pmd_address(vma, address, false, folio); >>> - >>> /* >>> * For THP, we have to assume the worse case ie pmd for invalidation. >>> * For hugetlb, it could be much worse if we need to do pud >>> @@ -1668,9 +1665,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>> mmu_notifier_invalidate_range_start(&range); >>> >>> while (page_vma_mapped_walk(&pvmw)) { >>> - /* Unexpected PMD-mapped THP? */ >>> - VM_BUG_ON_FOLIO(!pvmw.pte, folio); >>> - >>> /* >>> * If the folio is in an mlock()d vma, we must not swap it out. >>> */ >>> @@ -1682,6 +1676,21 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>> goto walk_done_err; >>> } >>> >>> + if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) { >>> + /* >>> + * We temporarily have to drop the PTL and start once >>> + * again from that now-PTE-mapped page table. >>> + */ >>> + split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, >>> + false, folio); >>> + flags &= ~TTU_SPLIT_HUGE_PMD; >>> + page_vma_mapped_walk_restart(&pvmw); >> >> If, for some reason, split_huge_pmd_locked() would fail, we would keep >> looping and never hit the VM_BUG_ON_FOLIO() below. Maybe we'd want to >> let split_huge_pmd_locked() return whether splitting succeeded, and >> handle that case differently? > > Hmm... after calling split_huge_pmd_locked(), we also do > "flags &= ~TTU_SPLIT_HUGE_PMD", preventing re-entry into this block, > then triggering the VM_BUG_ON_FOLIO() below if split_huge_pmd_locked() > fails, IIUC. > > What do you think? Missed that, you are right. -- Cheers, David / dhildenb