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Wed, 12 Jun 2024 23:55:43 -0700 (PDT) Received: from LancedeMBP.lan.lan ([2403:2c80:6::304e]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2c4c467ae5esm831570a91.47.2024.06.12.23.55.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Jun 2024 23:55:42 -0700 (PDT) From: Lance Yang To: baolin.wang@linux.alibaba.com, david@redhat.com Cc: ioworker0@gmail.com, 21cnbao@gmail.com, akpm@linux-foundation.org, 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 Subject: Re: [PATCH v7 4/4] mm/vmscan: avoid split lazyfree THP during shrink_folio_list() Date: Thu, 13 Jun 2024 14:55:21 +0800 Message-Id: <20240613065521.15960-1-ioworker0@gmail.com> X-Mailer: git-send-email 2.33.1 In-Reply-To: <20240610120809.66601-1-ioworker0@gmail.com> References: <20240610120809.66601-1-ioworker0@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: B87BB180005 X-Stat-Signature: gaobt4h11481rtzakh34bxs8ujer1zts X-HE-Tag: 1718261744-460372 X-HE-Meta: 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 agBXXJiz 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 10, 2024 at 8:08 PM Lance Yang wrote: > [...] > +static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma, > + unsigned long addr, pmd_t *pmdp, > + struct folio *folio) > +{ > + VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio); > + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio); > + > + struct mm_struct *mm = vma->vm_mm; > + int ref_count, map_count; > + pmd_t orig_pmd = *pmdp; > + struct page *page; > + > + if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd))) > + return false; > + > + page = pmd_page(orig_pmd); > + if (unlikely(page_folio(page) != folio)) > + return false; > + > + if (folio_test_dirty(folio) || pmd_dirty(orig_pmd)) { > + folio_set_swapbacked(folio); > + return false; > + } > + > + orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp); > + > + /* > + * Syncing against concurrent GUP-fast: > + * - clear PMD; barrier; read refcount > + * - inc refcount; barrier; read PMD > + */ > + smp_mb(); > + > + ref_count = folio_ref_count(folio); > + map_count = folio_mapcount(folio); > + > + /* > + * Order reads for folio refcount and dirty flag > + * (see comments in __remove_mapping()). > + */ > + smp_rmb(); > + > + /* > + * If the folio or its PMD is redirtied at this point, or if there > + * are unexpected references, we will give up to discard this folio > + * and remap it. > + * > + * The only folio refs must be one from isolation plus the rmap(s). > + */ > + if (folio_test_dirty(folio) || pmd_dirty(orig_pmd)) > + folio_set_swapbacked(folio); > + > + if (folio_test_swapbacked(folio) || ref_count != map_count + 1) { > + set_pmd_at(mm, addr, pmdp, orig_pmd); > + return false; > + } > + > + folio_remove_rmap_pmd(folio, page, vma); > + zap_deposited_table(mm, pmdp); > + add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR); > + if (vma->vm_flags & VM_LOCKED) > + mlock_drain_local(); > + folio_put(folio); > + > + return true; > +} [...] > diff --git a/mm/rmap.c b/mm/rmap.c > index b77f88695588..8e901636ade9 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1630,6 +1630,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, > enum ttu_flags flags = (enum ttu_flags)(long)arg; > unsigned long pfn; > unsigned long hsz = 0; > + bool pmd_mapped = false; > > /* > * When racing against e.g. zap_pte_range() on another cpu, > @@ -1676,16 +1677,24 @@ 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); > - continue; > + if (!pvmw.pte) { > + pmd_mapped = true; > + if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, > + folio)) > + goto walk_done; > + > + if (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); > + continue; > + } > } > > /* Unexpected PMD-mapped THP? */ > @@ -1813,7 +1822,12 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, > */ > if (unlikely(folio_test_swapbacked(folio) != > folio_test_swapcache(folio))) { > - WARN_ON_ONCE(1); > + /* > + * unmap_huge_pmd_locked() will unmark a > + * PMD-mapped folio as lazyfree if the folio or > + * its PMD was redirtied. > + */ > + WARN_ON_ONCE(!pmd_mapped); Damn it. I forgot to remap the folio to the page table since we've cleared the PTE here. But it seems like we neither need to do that, nor unmark a PMD-mapped folio as lazyfree in unmap_huge_pmd_locked(); the next block[1] will catch the folio and do the same thing. Hi David and Baolin, what do you think? > goto walk_done_err; > } [1] https://elixir.bootlin.com/linux/v6.10-rc3/source/mm/rmap.c#L1820 /* MADV_FREE page check */ if (!folio_test_swapbacked(folio)) { int ref_count, map_count; [...] /* * The only page refs must be one from isolation * plus the rmap(s) (dropped by discard:). */ if (ref_count == 1 + map_count && !folio_test_dirty(folio)) { dec_mm_counter(mm, MM_ANONPAGES); goto discard; } /* * If the folio was redirtied, it cannot be * discarded. Remap the page to page table. */ set_pte_at(mm, address, pvmw.pte, pteval); folio_set_swapbacked(folio); goto walk_done_err; } Thanks, Lance > > -- > 2.33.1 >