Subject: Re: [PATCH v2] powerpc/mm: Fix growth direction for hugepages mmaps with slice

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 16 Jan 2018 21:33:48 +0530

Christophe Leroy <[email-masked]> writes:

> An application running with libhugetlbfs fails to allocate
> additional pages to HEAP due to the hugemap being done
> inconditionally as topdown mapping:
>
> mmap(0x10080000, 1572864, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0) = 0x73e80000
> [...]
> mmap(0x74000000, 1048576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d80000
> munmap(0x73d80000, 1048576)             = 0
> [...]
> mmap(0x74000000, 1572864, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d00000
> munmap(0x73d00000, 1572864)             = 0
> [...]
> mmap(0x74000000, 1572864, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d00000
> munmap(0x73d00000, 1572864)             = 0
> [...]
>

Can you explain the failure details above. I am not sure I understand
what to read from the above output.

> As one can see from the above strace log, mmap() allocates further
> pages below the initial one.
>
> This patch fixes it by taking into account MAP_GROWSDOWN flag.

Rest of the kernel don't depend on that flag to select a topdown search
or not. So what is special with hugetlb? IF we select legacy mmap that
is when we select a bottomup search. Hugetlb on ppc64 always did a
topdown search.

>
> Fixes: d0f13e3c20b6f ("[POWERPC] Introduce address space "slices" ")
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  v2: Added missing include
>
>  arch/powerpc/mm/hugetlbpage.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 79e1378ee303..0eadf9f199de 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -19,6 +19,7 @@
>  #include <linux/moduleparam.h>
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
> +#include <linux/mman.h>
>  #include <asm/pgtable.h>
>  #include <asm/pgalloc.h>
>  #include <asm/tlb.h>
> @@ -558,7 +559,8 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  		return radix__hugetlb_get_unmapped_area(file, addr, len,
>  						       pgoff, flags);
>  #endif
> -	return slice_get_unmapped_area(addr, len, flags, mmu_psize, 1);
> +	return slice_get_unmapped_area(addr, len, flags, mmu_psize,
> +				       flags & MAP_GROWSDOWN);
>  }
>  #endif
>  
> -- 
> 2.13.3