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

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Message-ID: <89f33688-00dc-4073-90f4-657b6527cec4@lucifer.local>
Date: Fri, 9 May 2025 09:44:37 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, Liam.Howlett@...cle.com, vbabka@...e.cz,
        jannh@...gle.com, pfalcato@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, david@...hat.com, peterx@...hat.com,
        ryan.roberts@....com, mingo@...nel.org, libang.li@...group.com,
        maobibo@...ngson.cn, zhengqi.arch@...edance.com, baohua@...nel.org,
        anshuman.khandual@....com, willy@...radead.org, ioworker0@...il.com,
        yang@...amperecomputing.com, baolin.wang@...ux.alibaba.com,
        ziy@...dia.com, hughd@...gle.com
Subject: Re: [PATCH v2 0/2] Optimize mremap() for large folios

On Fri, May 09, 2025 at 10:57:05AM +0530, Dev Jain wrote:
>
>
> 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!

No problem, I've done this in the past myself haha (sometimes even faster
than you did :P), just a case of tweaking things to fit the mm process :)

Cheers, Lorenzo

>
> >
> > 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
> > >
>