Bug 704511

Description of problem:

If the total size of hugepages allocated on a system is over half of the total memory size, commitlimit becomes a negative number. That calculation is used in __vm_enough_memory() in mm/mmap.c.  This currently broken in the community.

Version-Release number of selected component (if applicable):

RHEL6.1 (and community)

How reproducible:

100%

Steps to Reproduce:
1. Allocate enough hugepages to consume over half of available memory
   For example "default_hugepagesz=1G hugepagesz=1G hugepages=64" on the
   linux kernel bootline.
2. After the system is booted, "cat /proc/meminfo".  Look at the "CommitLimit:"
   entry.  If is is a huge number (ie "CommitLimit:    737869762947802600 kB"
   it hit the problem.
  
Actual results:

CommitLimit:    737869762947802600 kB

Expected results:

A CommitLimit number less than the size of memory.

Additional info:

This is also broken in the community.  Reported it http://marc.info/?l=linux-kernel&m=130515303205772&w=2

If the total size of hugepages allocated on a system is over half of the total memory size, commitlimit becomes a negative number.
                    
What happens in fs/proc/meminfo.c is this calculation: 

        allowed = ((totalram_pages - hugetlb_total_pages())
                * sysctl_overcommit_ratio / 100) + total_swap_pages; 
          
The problem is that hugetlb_total_pages() is larger than totalram_pages resulting in a negative number.  Since allowed is an unsigned long the 
negative shows up as a big number. 
                       
A similar calculation occurs in __vm_enough_memory() in mm/mmap.c.
                    
A symptom of this problem is that /proc/meminfo prints a very large 
CommitLimit number.  
                    
CommitLimit:    737869762947802600 kB 
                          
To reproduce the problem reserve over half of memory as hugepages.  For 
example "default_hugepagesz=1G hugepagesz=1G hugepages=64". Then look at /proc/meminfo .

uv1-sys:~ # cat /proc/meminfo
MemTotal:       32395508 kB
MemFree:        32029276 kB
Buffers:            8656 kB
Cached:            89548 kB 
SwapCached:            0 kB 
Active:            55336 kB 
Inactive:          73916 kB 
Active(anon):      31220 kB 
Inactive(anon):       36 kB
Active(file):      24116 kB  
Inactive(file):    73880 kB 
Unevictable:           0 kB 
Mlocked:               0 kB  
SwapTotal:             0 kB  
SwapFree:              0 kB 
Dirty:              1692 kB 
Writeback:             0 kB 
AnonPages:         31132 kB 
Mapped:            15668 kB 
Shmem:               152 kB  
Slab:              70256 kB 
SReclaimable:      17148 kB 
SUnreclaim:        53108 kB 
KernelStack:        6536 kB 
PageTables:         3704 kB  
NFS_Unstable:          0 kB 
Bounce:                0 kB
WritebackTmp:          0 kB            
CommitLimit:    737869762947802600 kB
Committed_AS:     394044 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      713960 kB 
VmallocChunk:   34325764204 kB 
HardwareCorrupted:     0 kB 
HugePages_Total:      32 
HugePages_Free:       32  
HugePages_Rsvd:        0 
HugePages_Surp:        0 
Hugepagesize:    1048576 kB 
DirectMap4k:       16384 kB
DirectMap2M:     2064384 kB  
DirectMap1G:    65011712 kB