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

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Message-ID: <27619706-6758-4bc9-be40-528e949544f6@lucifer.local>
Date: Tue, 27 May 2025 17:32:51 +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 v3 0/2] Optimize mremap() for large folios

On Tue, May 27, 2025 at 09:56:37PM +0530, Dev Jain wrote:
>
> 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 : )

Yeah sorry maybe I wasn't clear. At any rate, I don't think we're miles off here
once we resolve the questions, so doesn't matter too much... :)

>
>
> > 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
> > >