Re: PageBlocks and Migrate Types

[Pankaj Suryawanshi <pankajssuryawanshi@gmail.com>]

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On Mon, Aug 26, 2019 at 12:34 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> 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.

it means if i have 4096 continuous pages = 1 pageblock
then all the 4096 pages of same type. but if any one page is different than
block type then ? it changed the block type or something else ?
>
>
> > Pageblocks and its type dynamically changes ?
>
> Yes as the purpose of the underlying memory for the block changes.
okay
>
> --
> Michal Hocko
> SUSE Labs

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