On Thu 21-05-15 09:18:59, Mike Kravetz wrote: > On 05/21/2015 06:27 AM, 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> > > Just for grins, I added this to my hugetlbfs fallocate stress testing > which really exercises hugetlb add and delete from page cache. > Everything is as expected. > > Tested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Thanks for your testing! -- Michal Hocko SUSE Labs -- 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>