Description of problem:

Eliminate hang when using /proc/sys/vm/drop_caches under heavy load on large system.

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

REHL4-U6

How reproducible:

Frequest but requires large system(~64GB) and multiple CPUs(~8) running
several(more then CPU count).

Steps to Reproduce:
1. Start several file system exercisers that create and/or read large enoug
files to exhaust memory in the pagecache.
2. "echo 3 > /proc/sys/vm /drop_caches" until system hangs
3. Capture AltSysrq-W ad verify all CPUs are stuck on the inode_lock.
  
Actual results:

System Hang.

Expected results:

Pagecache memory is freed without system hanging.

Additional info:

Back in RHEL4-U6 we backported the /proc/sys/vm/drop_caches
functionality from upstream to RHEL4.  Recently I encountered hang in
this code while creating 256GB files on a 64GB 4-core system and
dropping the pagecache at the same time.  The cause of the hang is
invalidate_list() calls invalidate_inode_pages() which calls
invalidate_mapping_pages() with the inode_lock held.  Since
invalidate_mapping_pages() calls cond_resched(), every CPU can try to
acquire the inode_lock if the time quantum of the process writing
to /proc/sys/vm/drop_caches expires.  So far I have only been able to
reproduce this problem when writing multiple huge files on every CPU and
"echo 3 > /proc/sys/vm/drop_caches" from a shell, but it can happen
randomly.

The attached patch fixes this problem by creating and calling a new
function invalidate_all_mapping_pages() which does not reschedule.  I
could not backport the upstream solution to RHEL4 because
invalidate_mapping_pages() is exported and the fix would break the kABI
but the fix is basically the same logic that is upstream.

The original BZ is 205722.