Le 16/01/2018 à 17:03, Aneesh Kumar K.V a écrit :

Christophe Leroy [off-list ref] writes:

quoted

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.

libhugetlbfs first requests an area of size 1.5Mbytes, at address 0x10080000
mmap() returns an area at address 0x73e80000

Then libhugetlbfs requests an additional area on top of that, ie at 
address 0x74000000, to expand the heap.
But mmap() returns an area at address 0x73d80000, ie under the previous 
area.

This is not the behaviour when using the generic (ie without mm_slices) 
hugepages code, and this is not what libhugetlbfs expects for expending 
the heap.

quoted

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.

The generic hugepage code does a bottomup search. First page is 
allocated at address 0x30000000 and following pages are allocated at 
requested addresses when requested, then libhugetlbfs has no issue 
expanding the heap when required.

quoted

Fixes: d0f13e3c20b6f ("[POWERPC] Introduce address space "slices" ")
Signed-off-by: Christophe Leroy <redacted>
---
  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