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

[Dev Jain]

On 10/06/25 5:41 pm, Lorenzo Stoakes wrote:
> On Tue, Jun 10, 2025 at 09:20:41AM +0530, Dev Jain wrote:
> > 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.
> Thanks this is good!
>
> > 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.
> Hmm, I thought people were struggling to get M3 to work with Asahi? :) or is
> this in a mac-based vm? I've not paid attention to recent developments.

I meant a Linux VM on Mac.

> > 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);
> >      }
> >
> > }
> Thanks for including! Very useful.
>
> > 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)
> It's nitty but these seem to be getting more and more abbreviated :) not a
> massive big deal however ;)
>
> > 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