On 03/20/2015 09:49 AM, Eric B Munson wrote: > Currently, pages which are marked as unevictable are protected from > compaction, but not from other types of migration. The POSIX real time > extension explicitly states that mlock() will prevent a major page > fault, but the spirit of this is that mlock() should give a process the > ability to control sources of latency, including minor page faults. > However, the mlock manpage only explicitly says that a locked page will > not be written to swap and this can cause some confusion. The > compaction code today does not give a developer who wants to avoid swap > but wants to have large contiguous areas available any method to achieve > this state. This patch introduces a sysctl for controlling compaction > behavior with respect to the unevictable lru. Users that demand no page > faults after a page is present can set compact_unevictable_allowed to 0 > and users who need the large contiguous areas can enable compaction on > locked memory by leaving the default value of 1. > > To illustrate this problem I wrote a quick test program that mmaps a > large number of 1MB files filled with random data. These maps are > created locked and read only. Then every other mmap is unmapped and I > attempt to allocate huge pages to the static huge page pool. When the > compact_unevictable_allowed sysctl is 0, I cannot allocate hugepages > after fragmenting memory. When the value is set to 1, allocations > succeed. > > Signed-off-by: Eric B Munson <emunson@xxxxxxxxxx> > Acked-by: Michal Hocko <mhocko@xxxxxxx> > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > Acked-by: Christoph Lameter <cl@xxxxxxxxx> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> Acked-by: Rik van Riel <riel@xxxxxxxxxx> -- All rights reversed -- 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>