On Fri, May 22, 2015 at 04:21:43PM +0200, Michal Hocko wrote: > On Thu 21-05-15 13:09:09, Johannes Weiner wrote: > > 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> > > Thanks! > > > 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. > > I've tried that (see the patch below) but it enlarged the code by almost > 1k > text data bss dec hex filename > 510323 74273 44440 629036 9992c mm/built-in.o.before > 511248 74273 44440 629961 99cc9 mm/built-in.o.after > > I am not sure the code size increase is worth it. Maybe we can reduce > the check to only PageCompound(page) as huge pages are no in the page > cache (yet). > That would be a more sensible route because it also avoids exposing the hugetlbfs destructor unnecessarily. -- Mel Gorman 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>