Re: [PATCH v2 3/3] mm: rmap: support batched unmapping for file large folios
From: Baolin Wang <baolin.wang@linux.alibaba.com>
Date: 2025-12-16 05:49:07
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On 2025/12/15 20:38, Lorenzo Stoakes wrote:
On Thu, Dec 11, 2025 at 04:16:56PM +0800, Baolin Wang wrote:quoted
Similar to folio_referenced_one(), we can apply batched unmapping for file large folios to optimize the performance of file folios reclamation. 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.Again, you must test on non-arm64 architectures and report the numbers for this also.
Yes, I've tested on the x86 machine, and will add the data in the commit message.
quoted
W/o patch: real 0m1.018s user 0m0.000s sys 0m1.018s W/ patch: real 0m0.249s user 0m0.000s sys 0m0.249s Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> --- mm/rmap.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)diff --git a/mm/rmap.c b/mm/rmap.c index ec232165c47d..4c9d5777c8da 100644 --- a/mm/rmap.c +++ b/mm/rmap.c@@ -1855,9 +1855,10 @@ 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))Why is it now ok to support file-backed batched unmapping when it wasn't in Barry's series (see [0])? You don't seem to be justifying this?
Barry's series[0] is merely aimed at optimizing lazyfree anonymous large folios and does not continue to optimize anonymous large folios or file-backed large folios at that point. Subsequently, Barry sent out a new patch (see [1]) to optimize anonymous large folios. As for file-backed large folios, the batched unmapping support is relatively simple, since we only need to clear the PTE entries for file-backed large folios.
[0]:https://lore.kernel.org/all/20250214093015.51024-4-21cnbao@gmail.com/T/#u (local)
[1] https://lore.kernel.org/all/20250513084620.58231-1-21cnbao@gmail.com/ (local)
quoted
return 1; + if (pte_unused(pte)) return 1;@@ -2223,7 +2224,7 @@ 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);Was this just a bug before?
Nope. Before this patch, we never supported batched unmapping for file-backed large folios, so the 'nr_pages' was always 1. After this patch, we should use the number of pages in this file-backed large folio.