Re: [PATCH] hugetlb: Do not account hugetlb pages as NR_FILE_PAGES

[Mel Gorman]

On Thu, May 21, 2015 at 03:27:22PM +0200, Michal Hocko wrote:
> 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.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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