Re: [PATCH v3 0/2] Optimize mremap() for large folios

[Dev Jain <dev.jain@arm.com>]

On 27/05/25 4:20 pm, Lorenzo Stoakes wrote:
> I seem to recall we agreed you'd hold off on this until the mprotect work
> was done :>) I see a lot of review there and was expecting a respin, unless


Oh, my interpretation was that you requested to hold this off for a bit to get
some review on the mprotect series first, apologies if you meant otherwise! I
posted that one or so week before so I thought enough time has passed : )


> I'm mistaken?
>
> At any rate we're in the merge window now so it's maybe not quite as
> important now :)
>
> We're pretty close to this being done anyway, just need some feedback on
> points raised (obviously David et al. may have further comments).
>
> Thanks, Lorenzo
>
> On Tue, May 27, 2025 at 01:20:47PM +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.
> OK this is more general than the stuff in 2/2, so you are doing this work
> for page-table split large folios also.
>
> I do think this _should_ be fine for that unless I've missed something. At
> any rate I've commented on this in 2/2.
>
>> 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);
>>      }
>>
>> }
>>
>> 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 | 57 +++++++++++++++++++++++++++++++++++++++--------------
>>   1 file changed, 42 insertions(+), 15 deletions(-)
>>
>> --
>> 2.30.2
>>