Re: [PATCH] mm: prohibit the last subpage from reusing the entire large folio

[Barry Song <21cnbao@gmail.com>]

On Fri, Mar 8, 2024 at 10:03 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 08.03.24 09:56, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > In a Copy-on-Write (CoW) scenario, the last subpage will reuse the entire
> > large folio, resulting in the waste of (nr_pages - 1) pages. This wasted
> > memory remains allocated until it is either unmapped or memory
> > reclamation occurs.
> >
> > The following small program can serve as evidence of this behavior
> >
> >   main()
> >   {
> >   #define SIZE 1024 * 1024 * 1024UL
> >           void *p = malloc(SIZE);
> >           memset(p, 0x11, SIZE);
> >           if (fork() == 0)
> >                   _exit(0);
> >           memset(p, 0x12, SIZE);
> >           printf("done\n");
> >           while(1);
> >   }
> >
> > For example, using a 1024KiB mTHP by:
> >   echo always > /sys/kernel/mm/transparent_hugepage/hugepages-1024kB/enabled
> >
> > (1) w/o the patch, it takes 2GiB,
> >
> > Before running the test program,
> >   / # free -m
> >                  total        used        free      shared  buff/cache   available
> >   Mem:            5754          84        5692           0          17        5669
> >   Swap:              0           0           0
> >
> >   / # /a.out &
> >   / # done
> >
> > After running the test program,
> >   / # free -m
> >                   total        used        free      shared  buff/cache   available
> >   Mem:            5754        2149        3627           0          19        3605
> >   Swap:              0           0           0
> >
> > (2) w/ the patch, it takes 1GiB only,
> >
> > Before running the test program,
> >   / # free -m
> >                   total        used        free      shared  buff/cache   available
> >   Mem:            5754          89        5687           0          17        5664
> >   Swap:              0           0           0
> >
> >   / # /a.out &
> >   / # done
> >
> > After running the test program,
> >   / # free -m
> >                  total        used        free      shared  buff/cache   available
> >   Mem:            5754        1122        4655           0          17        4632
> >   Swap:              0           0           0
> >
> > This patch migrates the last subpage to a small folio and immediately
> > returns the large folio to the system. It benefits both memory availability
> > and anti-fragmentation.
> >
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: Lance Yang <ioworker0@gmail.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >   mm/memory.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e17669d4f72f..0200bfc15f94 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3523,6 +3523,14 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
> >               folio_unlock(folio);
> >               return false;
> >       }
> > +     /*
> > +      * If the last subpage reuses the entire large folio, it would
> > +      * result in a waste of (nr_pages - 1) pages
> > +      */
> > +     if (folio_ref_count(folio) == 1 && folio_test_large(folio)) {
> > +             folio_unlock(folio);
> > +             return false;
> > +     }
> >       /*
> >        * Ok, we've got the only folio reference from our mapping
> >        * and the folio is locked, it's dark out, and we're wearing
>
>
> Why not simply:
>
> diff --git a/mm/memory.c b/mm/memory.c
> index e17669d4f72f7..46d286bd450c6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3498,6 +3498,10 @@ static vm_fault_t wp_page_shared(struct vm_fault
> *vmf, struct folio *folio)
>   static bool wp_can_reuse_anon_folio(struct folio *folio,
>                                      struct vm_area_struct *vma)
>   {
> +
> +       if (folio_test_large(folio))
> +               return false;
> +
>          /*
>           * We have to verify under folio lock: these early checks are
>           * just an optimization to avoid locking the folio and freeing
>
> We could only possibly succeed if we are the last one mapping a PTE
> either way. No we simply give up right away for the time being.

nice !

>
> --
> Cheers,
>
> David / dhildenb
>