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

[David Hildenbrand <david@redhat.com>]

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.

-- 
Cheers,

David / dhildenb