Re: PageBlocks and Migrate Types

[Michal Hocko <mhocko@kernel.org>]

On Mon 26-08-19 22:35:08, Pankaj Suryawanshi wrote:
> 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 ?

That really depends on the specific migrate type. CMA, ISOLATE migrate
types are all or nothing IIRC. I would have to check the code to tell
exactly when MOVABLE/UNMOVABLE pageblocks transitions are done.
steal_suitable_fallback sounds like a good start to look at.
-- 
Michal Hocko
SUSE Labs