Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation

[David Hildenbrand]

On 14.02.25 10:30, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> 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@xxxxxxxx>
> ---
>   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