On Tue, Apr 08, 2014 at 03:02:20PM -0400, Luiz Capitulino wrote: > HugeTLB is limited to allocating hugepages whose size are less than > MAX_ORDER order. This is so because HugeTLB allocates hugepages via > the buddy allocator. Gigantic pages (that is, pages whose size is > greater than MAX_ORDER order) have to be allocated at boottime. > > However, boottime allocation has at least two serious problems. First, > it doesn't support NUMA and second, gigantic pages allocated at > boottime can't be freed. > > This commit solves both issues by adding support for allocating gigantic > pages during runtime. It works just like regular sized hugepages, > meaning that the interface in sysfs is the same, it supports NUMA, > and gigantic pages can be freed. > > For example, on x86_64 gigantic pages are 1GB big. To allocate two 1G > gigantic pages on node 1, one can do: > > # echo 2 > \ > /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages > > And to free them all: > > # echo 0 > \ > /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages > > The one problem with gigantic page allocation at runtime is that it > can't be serviced by the buddy allocator. To overcome that problem, this > commit scans all zones from a node looking for a large enough contiguous > region. When one is found, it's allocated by using CMA, that is, we call > alloc_contig_range() to do the actual allocation. For example, on x86_64 > we scan all zones looking for a 1GB contiguous region. When one is found, > it's allocated by alloc_contig_range(). > > One expected issue with that approach is that such gigantic contiguous > regions tend to vanish as runtime goes by. The best way to avoid this for > now is to make gigantic page allocations very early during system boot, say > from a init script. Other possible optimization include using compaction, > which is supported by CMA but is not explicitly used by this commit. > > It's also important to note the following: > > 1. Gigantic pages allocated at boottime by the hugepages= command-line > option can be freed at runtime just fine > > 2. This commit adds support for gigantic pages only to x86_64. The > reason is that I don't have access to nor experience with other archs. > The code is arch indepedent though, so it should be simple to add > support to different archs > > 3. I didn't add support for hugepage overcommit, that is allocating > a gigantic page on demand when > /proc/sys/vm/nr_overcommit_hugepages > 0. The reason is that I don't > think it's reasonable to do the hard and long work required for > allocating a gigantic page at fault time. But it should be simple > to add this if wanted > > Signed-off-by: Luiz Capitulino <lcapitulino@xxxxxxxxxx> Looks good to me, thanks. Reviewed-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> -- 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>