Re: How cma allocation works ?

[Pankaj Suryawanshi <pankajssuryawanshi@gmail.com>]

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On Thu, Aug 22, 2019 at 6:32 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Wed 21-08-19 22:58:03, Pankaj Suryawanshi wrote:
> > Hello,
> >
> > Hard time to understand cma allocation how differs from normal
allocation ?
>
> The buddy allocator which is built for order-N sized allocations and it
> is highly optimized because it used from really hot paths. The allocator
> also involves memory reclaim to get memory when there is none
> immediatelly available.
>
> CMA allocator operates on a pre reserved physical memory range(s) and
> focuses on allocating areas that require physically contigous memory of
> larger sizes. Very broadly speaking. LWN usually contains nice writeups
> for many kernel internals. E.g. quick googling pointed to
https://lwn.net/Articles/486301/
>
> > I know theoretically how cma works.
> >
> > 1. How it reserved the memory (start pfn to end pfn) ? what is bitmap_*
> > functions ?
>
> Not sure what you are asking here TBH
I know it reserved memory at boot time from start pfn to end pfn, but when
i am requesting memory from cma it has different bitmap_*() in cma_alloc()
what they are ?
because we pass pfn and pfn+count to alloc_contig_range and pfn is come
from bitmap_*() function.
lets say i have reserved 100MB cma memory at boot time (strart pfn to end
pfn) and i am requesting allocation of 30MB from cma area then what is pfn
passed to alloc_contig_range() it is same as start pfn or
different.(calucaled by bitmap_*()) ?
>
> > 2. How alloc_contig_range() works ? it isolate all the pages including
> > unevictable pages, what is the practical work flow ? all this works with
> > virtual pages or physical pages ?
>
> Yes it isolates a specific physical contiguous (pfn) range, tries to
> move any used memory within that range and make it available for the
> caller.
what i understood here is that it isolate pages between range as
MIGRATE_ISOLATE, removed pages from buddy allocator including
allocated(movable/unevictable) pages. how unevictable pages isolated here ?
>
> > 3.what start_isolate_page_range() does ?
>
> There is some documentation for that function. Which part is not clear?
>
mention in above question

> > 4. what alloc_contig_migrate_range does() ?
>
> Have you checked the code? It simply tries to reclaim and/or migrate
> pages off the pfn range.
>
What is difference between migration, isolation and reclamation of pages ?

> > 5.what isolate_migratepages_range(), reclaim_clean_pages_from_list(),
> >  migrate_pages() and shrink_page_list() is doing ?
>
> Again, have you checked the code/comments? What exactly is not clear?
>
Why again migrate_isolate_range() ?
(reclaim_clean_pages_fron_list) if we are reclaiming only clean pages then
pages will not contiguous ? we have only clean pages which are not
contiguous ?
What is work of shrink_page_list() ? please explain all flow with taking
one allocation for example let say reserved cma 100MB and then request
allocation of 30MB then how all the flow/function will work ?
> > Please let me know the flow with simple example.
>
> Look at alloc_gigantic_page which is using the contiguous allocator to
> get 1GB physically contiguous memory ranges to be used for hugetlb
> pages.
>
Thanks
> HTH
> --
> Michal Hocko
> SUSE Labs

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