hugetlb: do not account hugetlb pages as NR_FILE_PAGES

hugetlb: do not account hugetlb pages as NR_FILE_PAGES

hugetlb pages uses add_to_page_cache to track shared mappings.  This is
OK from the data structure point of view but it is less so from the
NR_FILE_PAGES accounting:

	- huge pages are accounted as 4k which is clearly wrong
	- this counter is used as the amount of the reclaimable page
	  cache which is incorrect as well because hugetlb pages are
	  special and not reclaimable
	- the counter is then exported to userspace via /proc/meminfo
	  (in Cached:), /proc/vmstat and /proc/zoneinfo as
	  nr_file_pages which is confusing at least:
	  Cached:          8883504 kB
	  HugePages_Free:     8348
	  ...
	  Cached:          8916048 kB
	  HugePages_Free:      156
	  ...
	  thats 8192 huge pages allocated which is ~16G accounted as 32M

There are usually not that many huge pages in the system for this to
make any visible difference e.g.  by fooling __vm_enough_memory or
zone_pagecache_reclaimable.

Fix this by special casing huge pages in both __delete_from_page_cache
and __add_to_page_cache_locked.  replace_page_cache_page is currently
only used by fuse and that shouldn't touch hugetlb pages AFAICS but it
is more robust to check for special casing there as well.

Hugetlb pages shouldn't get to any other paths where we do accounting:
	- migration - we have a special handling via
	  hugetlbfs_migrate_page
	- shmem - doesn't handle hugetlb pages directly even for
	  SHM_HUGETLB resp. MAP_HUGETLB
	- swapcache - hugetlb is not swapable

This has a user visible effect but I believe it is reasonable because the
previously exported number is simply bogus.

An alternative would be to account hugetlb pages with their real size and
treat them similar to shmem.  But this has some drawbacks.

First we would have to special case in kernel users of NR_FILE_PAGES and
considering how hugetlb is special we would have to do it everywhere.  We
do not want Cached exported by /proc/meminfo to include it because the
value would be even more misleading.

__vm_enough_memory and zone_pagecache_reclaimable would have to do the
same thing because those pages are simply not reclaimable.  The correction
is even not trivial because we would have to consider all active hugetlb
page sizes properly.  Users of the counter outside of the kernel would
have to do the same.

So the question is why to account something that needs to be basically
excluded for each reasonable usage.  This doesn't make much sense to me.

It seems that this has been broken since hugetlb was introduced but I
haven't checked the whole history.

[akpm@linux-foundation.org: tweak comments]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Tested-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>