[PATCH v3 4/4] mm: Avoid splitting pmd for lazyfree pmd-mapped THP in try_to_unmap

[Baolin Wang]

On 2025/1/15 11:38, Barry Song wrote:
> From: Barry Song <v-songbaohua at oppo.com>
> 
> The try_to_unmap_one() function currently handles PMD-mapped THPs
> inefficiently. It first splits the PMD into PTEs, copies the dirty
> state from the PMD to the PTEs, iterates over the PTEs to locate
> the dirty state, and then marks the THP as swap-backed. This process
> involves unnecessary PMD splitting and redundant iteration. Instead,
> this functionality can be efficiently managed in
> __discard_anon_folio_pmd_locked(), avoiding the extra steps and
> improving performance.
> 
> The following microbenchmark redirties folios after invoking MADV_FREE,
> then measures the time taken to perform memory reclamation (actually
> set those folios swapbacked again) on the redirtied folios.
> 
>   #include <stdio.h>
>   #include <sys/mman.h>
>   #include <string.h>
>   #include <time.h>
> 
>   #define SIZE 128*1024*1024  // 128 MB
> 
>   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);
>   		/* redirty after MADV_FREE */
>   		memset((void *)p, 1, SIZE);
> 
> 		clock_t start_time = clock();
>   		madvise((void *)p, SIZE, MADV_PAGEOUT);
>   		clock_t end_time = clock();
> 
>   		double elapsed_time = (double)(end_time - start_time) / CLOCKS_PER_SEC;
>   		printf("Time taken by reclamation: %f seconds\n", elapsed_time);
> 
>   		munmap((void *)p, SIZE);
>   	}
>   	return 0;
>   }
> 
> Testing results are as below,
> w/o patch:
> ~ # ./a.out
> Time taken by reclamation: 0.007300 seconds
> Time taken by reclamation: 0.007226 seconds
> Time taken by reclamation: 0.007295 seconds
> Time taken by reclamation: 0.007731 seconds
> Time taken by reclamation: 0.007134 seconds
> Time taken by reclamation: 0.007285 seconds
> Time taken by reclamation: 0.007720 seconds
> Time taken by reclamation: 0.007128 seconds
> Time taken by reclamation: 0.007710 seconds
> Time taken by reclamation: 0.007712 seconds
> Time taken by reclamation: 0.007236 seconds
> Time taken by reclamation: 0.007690 seconds
> Time taken by reclamation: 0.007174 seconds
> Time taken by reclamation: 0.007670 seconds
> Time taken by reclamation: 0.007169 seconds
> Time taken by reclamation: 0.007305 seconds
> Time taken by reclamation: 0.007432 seconds
> Time taken by reclamation: 0.007158 seconds
> Time taken by reclamation: 0.007133 seconds
> …
> 
> w/ patch
> 
> ~ # ./a.out
> Time taken by reclamation: 0.002124 seconds
> Time taken by reclamation: 0.002116 seconds
> Time taken by reclamation: 0.002150 seconds
> Time taken by reclamation: 0.002261 seconds
> Time taken by reclamation: 0.002137 seconds
> Time taken by reclamation: 0.002173 seconds
> Time taken by reclamation: 0.002063 seconds
> Time taken by reclamation: 0.002088 seconds
> Time taken by reclamation: 0.002169 seconds
> Time taken by reclamation: 0.002124 seconds
> Time taken by reclamation: 0.002111 seconds
> Time taken by reclamation: 0.002224 seconds
> Time taken by reclamation: 0.002297 seconds
> Time taken by reclamation: 0.002260 seconds
> Time taken by reclamation: 0.002246 seconds
> Time taken by reclamation: 0.002272 seconds
> Time taken by reclamation: 0.002277 seconds
> Time taken by reclamation: 0.002462 seconds

Good result.

> This patch significantly speeds up try_to_unmap_one() by allowing it
> to skip redirtied THPs without splitting the PMD.
> 
> Suggested-by: Baolin Wang <baolin.wang at linux.alibaba.com>
> Suggested-by: Lance Yang <ioworker0 at gmail.com>
> Signed-off-by: Barry Song <v-songbaohua at oppo.com>

LGTM.
Reviewed-by: Baolin Wang <baolin.wang at linux.alibaba.com>