Re: PageBlocks and Migrate Types

[Pankaj Suryawanshi]

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 and its type dynamically changes ?
>
>
> HTH
> --
> Michal Hocko
> SUSE Labs