Re: [PATCH v2 0/2] Optimize mremap() for large folios

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

On 09/05/25 12:05 am, Lorenzo Stoakes wrote:
> Dev - a general comment here - but let's slow things down a little please
> :)
> 
> The mprotect() version of this is still outstanding fixes and likely will
> need quite a bit of checking before we can ensure it's stabilised.
> 
> And now we have this mremap() series as well which also has had quite a few
> quite significant issues that have needed addressing.
> 
> So can we try to focus on one at a time, and really try to nail down the
> series before moving on to the next?
> 
> We also have outstanding review on the v1, which has now been split, which
> does happen sometimes but perhaps suggests that it'd work better if you
> waited a couple days or such to ensure things are settled before sending a
> new version when there's quite a bit of feedback?

Sure, I should have waited my bad, I usually do, this time I was in a 
haste with both series for no reason :( thanks for your detailed replies 
btw!

> 
> This isn't a criticism really, sorry I don't mean to sound negative or such
> - but this is more a process thing so we reviewers can keep up with things,
> keep things rolling, and ensure you get your changes merged asap :)
> 
> Thanks, Lorenzo
> 
> On Wed, May 07, 2025 at 11:32:54AM +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.
>>
>> Mapping 512K of memory, memsetting it, remapping it to src + 512K, 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).
>>
>> 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) // 512 KB
>>
>> 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);
>>      }
>>
>> }
>>
>> 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
>>
>>   include/linux/pgtable.h | 29 ++++++++++++++++++++++
>>   mm/mremap.c             | 54 +++++++++++++++++++++++++++++------------
>>   2 files changed, 68 insertions(+), 15 deletions(-)
>>
>> --
>> 2.30.2
>>