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

[Jisheng Zhang]

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]?

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