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

Naoya Horiguchi
2015-05-22T05:10:23Z

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@suse.cz>

looks good to me,

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>