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: Johannes Weiner <hannes@xxxxxxxxxxx> This makes a lot of sense to me. The only thing I worry about is the proliferation of PageHuge(), a function call, in relatively hot paths. Naoya-san, would there be a strong reason to make this function a static inline in the header? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>