On Thu 22-08-19 23:54:19, Pankaj Suryawanshi wrote: > On Thu, Aug 22, 2019 at 6:22 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Wed 21-08-19 22:23:44, Pankaj Suryawanshi wrote: > > > Hello, > > > > > > 1. What are Pageblocks and migrate types(MIGRATE_CMA) in Linux memory ? > > > > Pageblocks are a simple grouping of physically contiguous pages with > > common set of flags. I haven't checked closely recently so I might > > misremember but my recollection is that only the migrate type is stored > > there. Normally we would store that information into page flags but > > there is not enough room there. > > > > MIGRATE_CMA represent pages allocated for the CMA allocator. There are > > other migrate types denoting unmovable/movable allocations or pages that > > are isolated from the page allocator. > > > > Very broadly speaking, the migrate type groups pages with similar > > movability properties to reduce fragmentation that compaction cannot > > do anything about because there are objects of different properti > > around. Please note that pageblock might contain objects of a different > > migrate type in some cases (e.g. low on memory). > > > > Have a look at gfpflags_to_migratetype and how the gfp mask is converted > > to a migratetype for the allocation. Also follow different MIGRATE_$TYPE > > to see how it is used in the code. > > > > > How many movable/unmovable pages are defined by default? > > > > There is nothing like that. It depends on how many objects of a specific > > type are allocated. > > > It means that it started creating pageblocks after allocation of > different objects, but from which block it allocate initially when > there is nothing like pageblocks ? (when memory subsystem up) Pageblocks are just a way to group physically contiguous pages. They just exist along with the physically contiguous memory. The migrate type for most of the memory is set to MIGRATE_MOVABLE. Portion of the memory might be reserved by CMA then that memory has MIGRATE_CMA. Following set_pageblock_migratetype call paths will give you a good picture. > Pageblocks and its type dynamically changes ? Yes as the purpose of the underlying memory for the block changes. -- Michal Hocko SUSE Labs
