Re: [PATCH] mm: free reserved area's memmap if possiable

[zhouxianrong]

On 2017/3/1 18:41, Jisheng Zhang wrote:
> Add Chen, Catalin
>
> On Thu, 16 Feb 2017 09:11:29 +0800 zhouxianrong wrote:
> > On 2017/2/15 15:10, Ard Biesheuvel wrote:
> > > On 15 February 2017 at 01:44, zhouxianrong wrote:
> > > > On 2017/2/14 17:03, Ard Biesheuvel wrote:
> > > > > On 14 February 2017 at 06:53,  <zhouxianrong@xxxxxxxxxx> wrote:
> > > > > > From: zhouxianrong <zhouxianrong@xxxxxxxxxx>
> > > > > >
> > > > > > just like freeing no-map area's memmap (gaps of memblock.memory)
> > > > > > we could free reserved area's memmap (areas of memblock.reserved)
> > > > > > as well only when user of reserved area indicate that we can do
> > > > > > this in drivers. that is, user of reserved area know how to
> > > > > > use the reserved area who could not memblock_free or free_reserved_xxx
> > > > > > the reserved area and regard the area as raw pfn usage by kernel.
> > > > > > the patch supply a way to users who want to utilize the memmap
> > > > > > memory corresponding to raw pfn reserved areas as many as possible.
> > > > > > users can do this by memblock_mark_raw_pfn interface which mark the
> > > > > > reserved area as raw pfn and tell free_unused_memmap that this area's
> > > > > > memmap could be freeed.
> > > > >
> > > > > Could you give an example how much memory we actually recover by doing
> > > > > this? I understand it depends on the size of the reserved regions, but
> > > > > I'm sure you have an actual example that inspired you to write this
> > > > > patch.
> > > >
> > > >
> > > > i did statistics in our platform, the memmap of reserved region that can be
> > > > freed
> > > > is about 6MB. it's fewer.
>
> <...>
>
> > > > > In any case, it is good to emphasize that on 4 KB pagesize kernels, we
> > > > > will only free multiples of 8 MB that are 8 MB aligned, resulting in
> > > > > 128 KB of memmap backing to be released.
> > > >
> > > >
> > > >
> > > > > > +               if (start < end)
> > > > > > +                       free_memmap(start, end);
> > > > > > +       }
> > > > > >  }
> > > > > >  #endif /* !CONFIG_SPARSEMEM_VMEMMAP */
> > > > > >
> > > > > > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > > > > > index 5b759c9..9f8d277 100644
> > > > > > --- a/include/linux/memblock.h
> > > > > > +++ b/include/linux/memblock.h
> > > > > > @@ -26,6 +26,7 @@ enum {
> > > > > >         MEMBLOCK_HOTPLUG        = 0x1,  /* hotpluggable region */
> > > > > >         MEMBLOCK_MIRROR         = 0x2,  /* mirrored region */
> > > > > >         MEMBLOCK_NOMAP          = 0x4,  /* don't add to kernel direct
> > > > > > mapping */
> > > > > > +       MEMBLOCK_RAW_PFN        = 0x8,  /* region whose memmap never be
> > > > > > used */
> > > > >
> > > > >
> > > > > I think we should be *very* careful about the combinatorial explosion
> > > > > that results when combining all these flags, given that this is not a
> > > > > proper enum but a bit field.
> > > > >
> > > > > In any case, the generic memblock change should be in a separate patch
> > > > > from the arm64 change.
> > > >
> > > >
> > > > MEMBLOCK_RAW_PFN and MEMBLOCK_NOMAP can not be set at the same time
> > >
> > > They should not. But if I call  memblock_mark_raw_pfn() on a
> > > MEMBLOCK_NOMAP region, it will have both flags set.
> > >
> > > In summary, I don't think we need this patch. And if you can convince
> > > us otherwise, you should really be more methodical and explicit in
> > > implementing this RAW_PFN flag, not add it as a byproduct of the arch
> > > code that uses it. Also, you should explain how RAW_PFN relates to
> > > NOMAP, and ensure that RAW_PFN and NOMAP regions don't intersect if
> > > that is an unsupported combination.
> >
> > yes, setting both MEMBLOCK_RAW_PFN and MEMBLOCK_NOMAP could meet some problems
> > when gaps of memblock.memory intersect memblock.reserved. if they do not intersect,
> > that's ok. so as you said this should be carefully considered.
> >
> > as you think this patch is not needed because, i have showed my idea, it's enough, thanks!
>
> we are also interested in this area.
>
> Just curious, is this patch to "free the vmemmap holes" mentioned by
> by Catalin in [1]?

free the vmemmap of reserved memblock (other than no-map regions) whose driver owner know
it is never be used.

> [1]http://lkml.iu.edu/hypermail/linux/kernel/1604.1/03036.html
>
> .