Re: PageBlocks and Migrate Types

[Michal Hocko <mhocko@kernel.org>]

On Thu 22-08-19 23:54:19, Pankaj Suryawanshi wrote:
> On Thu, Aug 22, 2019 at 6:22 PM Michal Hocko <mhocko@kernel.org> 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