[PATCH v4 0/2] Optimize mremap() for large folios

[Dev Jain]

Currently move_ptes() iterates through ptes one by one. If the underlying
folio mapped by the ptes is large, we can process those ptes in a batch
using folio_pte_batch(), thus clearing and setting the PTEs in one go.
For arm64 specifically, this results in a 16x reduction in the number of
ptep_get() calls (since on a contig block, ptep_get() on arm64 will iterate
through all 16 entries to collect a/d bits), and we also elide extra TLBIs
through get_and_clear_full_ptes, replacing ptep_get_and_clear.

Mapping 1M of memory with 64K folios, memsetting it, remapping it to
src + 1M, and munmapping it 10,000 times, the average execution time
reduces from 1.9 to 1.2 seconds, giving a 37% performance optimization,
on Apple M3 (arm64). No regression is observed for small folios.

The patchset is based on mm-unstable (6ebffe676fcf).

Test program for reference:

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>

#define SIZE (1UL << 20) // 1M

int main(void) {
    void *new_addr, *addr;

    for (int i = 0; i < 10000; ++i) {
        addr = mmap((void *)(1UL << 30), SIZE, PROT_READ | PROT_WRITE,
                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (addr == MAP_FAILED) {
                perror("mmap");
                return 1;
        }
        memset(addr, 0xAA, SIZE);

        new_addr = mremap(addr, SIZE, SIZE, MREMAP_MAYMOVE | MREMAP_FIXED, addr + SIZE);
        if (new_addr != (addr + SIZE)) {
                perror("mremap");
                return 1;
        }
        munmap(new_addr, SIZE);
    }

}

v3->v4:
 - Remove comment above mremap_folio_pte_batch, improve patch description
   differentiating between folio splitting and pagetable splitting
v2->v3:
 - Refactor mremap_folio_pte_batch, drop maybe_contiguous_pte_pfns, fix
   indentation (Lorenzo), fix cover letter description (512K -> 1M)

v1->v2:
 - Expand patch descriptions, move pte declarations to a new line,
   reduce indentation in patch 2 by introducing mremap_folio_pte_batch(),
   fix loop iteration (Lorenzo)
 - Merge patch 2 and 3 (Anshuman, Lorenzo)
 - Fix maybe_contiguous_pte_pfns (Willy)

Dev Jain (2):
  mm: Call pointers to ptes as ptep
  mm: Optimize mremap() by PTE batching

 mm/mremap.c | 58 ++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 42 insertions(+), 16 deletions(-)

-- 
2.30.2