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Message-ID: <a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com>
Date: Tue, 24 Jun 2025 14:55:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org
Cc: baolin.wang@...ux.alibaba.com, chrisl@...nel.org, ioworker0@...il.com,
kasong@...cent.com, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
lorenzo.stoakes@...cle.com, ryan.roberts@....com, v-songbaohua@...o.com,
x86@...nel.org, ying.huang@...el.com, zhengtangquan@...o.com
Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large
folios during reclamation
On 14.02.25 10:30, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
>
> Currently, the PTEs and rmap of a large folio are removed one at a time.
> This is not only slow but also causes the large folio to be unnecessarily
> added to deferred_split, which can lead to races between the
> deferred_split shrinker callback and memory reclamation. This patch
> releases all PTEs and rmap entries in a batch.
> Currently, it only handles lazyfree large folios.
>
> The below microbench tries to reclaim 128MB lazyfree large folios
> whose sizes are 64KiB:
>
> #include <stdio.h>
> #include <sys/mman.h>
> #include <string.h>
> #include <time.h>
>
> #define SIZE 128*1024*1024 // 128 MB
>
> unsigned long read_split_deferred()
> {
> FILE *file = fopen("/sys/kernel/mm/transparent_hugepage"
> "/hugepages-64kB/stats/split_deferred", "r");
> if (!file) {
> perror("Error opening file");
> return 0;
> }
>
> unsigned long value;
> if (fscanf(file, "%lu", &value) != 1) {
> perror("Error reading value");
> fclose(file);
> return 0;
> }
>
> fclose(file);
> return value;
> }
>
> int main(int argc, char *argv[])
> {
> while(1) {
> volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>
> memset((void *)p, 1, SIZE);
>
> madvise((void *)p, SIZE, MADV_FREE);
>
> clock_t start_time = clock();
> unsigned long start_split = read_split_deferred();
> madvise((void *)p, SIZE, MADV_PAGEOUT);
> clock_t end_time = clock();
> unsigned long end_split = read_split_deferred();
>
> double elapsed_time = (double)(end_time - start_time) / CLOCKS_PER_SEC;
> printf("Time taken by reclamation: %f seconds, split_deferred: %ld\n",
> elapsed_time, end_split - start_split);
>
> munmap((void *)p, SIZE);
> }
> return 0;
> }
>
> w/o patch:
> ~ # ./a.out
> Time taken by reclamation: 0.177418 seconds, split_deferred: 2048
> Time taken by reclamation: 0.178348 seconds, split_deferred: 2048
> Time taken by reclamation: 0.174525 seconds, split_deferred: 2048
> Time taken by reclamation: 0.171620 seconds, split_deferred: 2048
> Time taken by reclamation: 0.172241 seconds, split_deferred: 2048
> Time taken by reclamation: 0.174003 seconds, split_deferred: 2048
> Time taken by reclamation: 0.171058 seconds, split_deferred: 2048
> Time taken by reclamation: 0.171993 seconds, split_deferred: 2048
> Time taken by reclamation: 0.169829 seconds, split_deferred: 2048
> Time taken by reclamation: 0.172895 seconds, split_deferred: 2048
> Time taken by reclamation: 0.176063 seconds, split_deferred: 2048
> Time taken by reclamation: 0.172568 seconds, split_deferred: 2048
> Time taken by reclamation: 0.171185 seconds, split_deferred: 2048
> Time taken by reclamation: 0.170632 seconds, split_deferred: 2048
> Time taken by reclamation: 0.170208 seconds, split_deferred: 2048
> Time taken by reclamation: 0.174192 seconds, split_deferred: 2048
> ...
>
> w/ patch:
> ~ # ./a.out
> Time taken by reclamation: 0.074231 seconds, split_deferred: 0
> Time taken by reclamation: 0.071026 seconds, split_deferred: 0
> Time taken by reclamation: 0.072029 seconds, split_deferred: 0
> Time taken by reclamation: 0.071873 seconds, split_deferred: 0
> Time taken by reclamation: 0.073573 seconds, split_deferred: 0
> Time taken by reclamation: 0.071906 seconds, split_deferred: 0
> Time taken by reclamation: 0.073604 seconds, split_deferred: 0
> Time taken by reclamation: 0.075903 seconds, split_deferred: 0
> Time taken by reclamation: 0.073191 seconds, split_deferred: 0
> Time taken by reclamation: 0.071228 seconds, split_deferred: 0
> Time taken by reclamation: 0.071391 seconds, split_deferred: 0
> Time taken by reclamation: 0.071468 seconds, split_deferred: 0
> Time taken by reclamation: 0.071896 seconds, split_deferred: 0
> Time taken by reclamation: 0.072508 seconds, split_deferred: 0
> Time taken by reclamation: 0.071884 seconds, split_deferred: 0
> Time taken by reclamation: 0.072433 seconds, split_deferred: 0
> Time taken by reclamation: 0.071939 seconds, split_deferred: 0
> ...
>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
> mm/rmap.c | 72 ++++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 50 insertions(+), 22 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 89e51a7a9509..8786704bd466 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1781,6 +1781,25 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page,
> #endif
> }
>
> +/* We support batch unmapping of PTEs for lazyfree large folios */
> +static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
> + struct folio *folio, pte_t *ptep)
> +{
> + const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> + int max_nr = folio_nr_pages(folio);
Let's assume we have the first page of a folio mapped at the last page
table entry in our page table.
What prevents folio_pte_batch() from reading outside the page table?
--
Cheers,
David / dhildenb
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