[LinuxPPS] 24 Jan 16:07 - fix hugepages leak due to pagetable page sharing

[linuxpps: Larry Woodman <>]

fix hugepages leak due to pagetable page sharing  
  
The shared page table code for hugetlb memory on x86 and x86_64  
is causing a leak. When a user of hugepages exits using this code  
the system leaks some of the hugepages.  
  
-------------------------------------------------------  
Part of /proc/meminfo just before database startup:  
HugePages_Total: 5500  
HugePages_Free: 5500  
HugePages_Rsvd: 0  
Hugepagesize: 2048 kB  
  
Just before shutdown:  
HugePages_Total: 5500  
HugePages_Free: 4475  
HugePages_Rsvd: 0  
Hugepagesize: 2048 kB  
  
After shutdown:  
HugePages_Total: 5500  
HugePages_Free: 4988  
HugePages_Rsvd:  
0 Hugepagesize: 2048 kB  
----------------------------------------------------------  
  
The problem occurs durring a fork, in copy_hugetlb_page_range(). It  
locates the dst_pte using huge_pte_alloc(). Since huge_pte_alloc() calls  
huge_pmd_share() it will share the pmd page if can, yet the main loop in  
copy_hugetlb_page_range() does a get_page() on every hugepage. This is a  
violation of the shared hugepmd pagetable protocol and creates additional  
referenced to the hugepages causing a leak when the unmap of the VMA  
occurs. We can skip the entire replication of the ptes when the hugepage  
pagetables are shared. The attached patch skips copying the ptes and the  
get_page() calls if the hugetlbpage pagetable is shared.  
  
[akpm at linux-foundation.org: coding-style cleanups]  
Signed-off-by: Larry Woodman   
Signed-off-by: Adam Litke   
Cc: Badari Pulavarty   
Cc: Ken Chen   
Cc: David Gibson   
Cc: William Lee Irwin III   
Signed-off-by: Andrew Morton   
Signed-off-by: Linus Torvalds   
  
mm/hugetlb.c

URL: http://gitweb.enneenne.com/?p=linuxpps;a=commit;h=c5c99429fa57dcf6e05203ebe3676db1ec646793