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Tue, 06 Jan 2026 17:46:02 -0800 (PST) Date: Wed, 7 Jan 2026 01:46:01 +0000 From: Wei Yang To: Barry Song <21cnbao@gmail.com> Cc: Wei Yang , Baolin Wang , akpm@linux-foundation.org, david@kernel.org, catalin.marinas@arm.com, will@kernel.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, riel@surriel.com, harry.yoo@oracle.com, jannh@google.com, willy@infradead.org, dev.jain@arm.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 5/5] mm: rmap: support batched unmapping for file large folios Message-ID: <20260107014601.dxvq6b7ljgxwg7iu@master> Reply-To: Wei Yang References: <142919ac14d3cf70cba370808d85debe089df7b4.1766631066.git.baolin.wang@linux.alibaba.com> <20260106132203.kdxfvootlkxzex2l@master> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) On Wed, Jan 07, 2026 at 10:29:25AM +1300, Barry Song wrote: >On Wed, Jan 7, 2026 at 2:22 AM Wei Yang wrote: >> >> On Fri, Dec 26, 2025 at 02:07:59PM +0800, Baolin Wang wrote: >> >Similar to folio_referenced_one(), we can apply batched unmapping for file >> >large folios to optimize the performance of file folios reclamation. >> > >> >Barry previously implemented batched unmapping for lazyfree anonymous large >> >folios[1] and did not further optimize anonymous large folios or file-backed >> >large folios at that stage. As for file-backed large folios, the batched >> >unmapping support is relatively straightforward, as we only need to clear >> >the consecutive (present) PTE entries for file-backed large folios. >> > >> >Performance testing: >> >Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and try to >> >reclaim 8G file-backed folios via the memory.reclaim interface. I can observe >> >75% performance improvement on my Arm64 32-core server (and 50%+ improvement >> >on my X86 machine) with this patch. >> > >> >W/o patch: >> >real 0m1.018s >> >user 0m0.000s >> >sys 0m1.018s >> > >> >W/ patch: >> >real 0m0.249s >> >user 0m0.000s >> >sys 0m0.249s >> > >> >[1] https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u >> >Reviewed-by: Ryan Roberts >> >Acked-by: Barry Song >> >Signed-off-by: Baolin Wang >> >--- >> > mm/rmap.c | 7 ++++--- >> > 1 file changed, 4 insertions(+), 3 deletions(-) >> > >> >diff --git a/mm/rmap.c b/mm/rmap.c >> >index 985ab0b085ba..e1d16003c514 100644 >> >--- a/mm/rmap.c >> >+++ b/mm/rmap.c >> >@@ -1863,9 +1863,10 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >> > end_addr = pmd_addr_end(addr, vma->vm_end); >> > max_nr = (end_addr - addr) >> PAGE_SHIFT; >> > >> >- /* We only support lazyfree batching for now ... */ >> >- if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >> >+ /* We only support lazyfree or file folios batching for now ... */ >> >+ if (folio_test_anon(folio) && folio_test_swapbacked(folio)) >> > return 1; >> >+ >> > if (pte_unused(pte)) >> > return 1; >> > >> >@@ -2231,7 +2232,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >> > * >> > * See Documentation/mm/mmu_notifier.rst >> > */ >> >- dec_mm_counter(mm, mm_counter_file(folio)); >> >+ add_mm_counter(mm, mm_counter_file(folio), -nr_pages); >> > } >> > discard: >> > if (unlikely(folio_test_hugetlb(folio))) { >> >-- >> >2.47.3 >> > >> >> Hi, Baolin >> >> When reading your patch, I come up one small question. >> >> Current try_to_unmap_one() has following structure: >> >> try_to_unmap_one() >> while (page_vma_mapped_walk(&pvmw)) { >> nr_pages = folio_unmap_pte_batch() >> >> if (nr_pages = folio_nr_pages(folio)) >> goto walk_done; >> } >> >> I am thinking what if nr_pages > 1 but nr_pages != folio_nr_pages(). >> >> If my understanding is correct, page_vma_mapped_walk() would start from >> (pvmw->address + PAGE_SIZE) in next iteration, but we have already cleared to >> (pvmw->address + nr_pages * PAGE_SIZE), right? >> >> Not sure my understanding is correct, if so do we have some reason not to >> skip the cleared range? > >I don’t quite understand your question. For nr_pages > 1 but not equal >to nr_pages, page_vma_mapped_walk will skip the nr_pages - 1 PTEs inside. > >take a look: > >next_pte: > do { > pvmw->address += PAGE_SIZE; > if (pvmw->address >= end) > return not_found(pvmw); > /* Did we cross page table boundary? */ > if ((pvmw->address & (PMD_SIZE - PAGE_SIZE)) == 0) { > if (pvmw->ptl) { > spin_unlock(pvmw->ptl); > pvmw->ptl = NULL; > } > pte_unmap(pvmw->pte); > pvmw->pte = NULL; > pvmw->flags |= PVMW_PGTABLE_CROSSED; > goto restart; > } > pvmw->pte++; > } while (pte_none(ptep_get(pvmw->pte))); > Yes, we do it in page_vma_mapped_walk() now. Since they are pte_none(), they will be skipped. I mean maybe we can skip it in try_to_unmap_one(), for example: diff --git a/mm/rmap.c b/mm/rmap.c index 9e5bd4834481..ea1afec7c802 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2250,6 +2250,10 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, */ if (nr_pages == folio_nr_pages(folio)) goto walk_done; + else { + pvmw.address += PAGE_SIZE * (nr_pages - 1); + pvmw.pte += nr_pages - 1; + } continue; walk_abort: ret = false; Not sure this is reasonable. -- Wei Yang Help you, Help me