Re: [PATCH v3 0/4] mm: fix the "counter.sh" failure for libhugetlbfs

[Huang Shijie <shijie.huang@arm.com>]

On Wed, Dec 07, 2016 at 11:02:38PM +0800, Michal Hocko wrote:
> On Tue 06-12-16 18:03:59, Huang Shijie wrote:
> > On Mon, Dec 05, 2016 at 05:31:01PM +0800, Michal Hocko wrote:
> > > On Mon 05-12-16 17:17:07, Huang Shijie wrote:
> > > [...]
> > > >    The failure is caused by:
> > > >     1) kernel fails to allocate a gigantic page for the surplus case.
> > > >        And the gather_surplus_pages() will return NULL in the end.
> > > > 
> > > >     2) The condition checks for some functions are wrong:
> > > >         return_unused_surplus_pages()
> > > >         nr_overcommit_hugepages_store()
> > > >         hugetlb_overcommit_handler()
>add the  > > 
> > > OK, so how is this any different from gigantic (1G) hugetlb pages on
> > I think there is no different from gigantic (1G) hugetlb pages on
> > x86_64. Do anyone ever tested the 1G hugetlb pages in x86_64 with the "counter.sh"
> > before? 
> 
> I suspect nobody has because the gigantic page support is still somehow
> coarse and from a quick look into the code we only support pre-allocated
Yes, the x86_64 even does not support the gigantic page.
The default x86_64_defconfig does not enable the CONFIG_CMA.

I enabled the CONFIG_CMA, and did the test for gigantic page in x86_64.
(I appended "hugepagesz=1G hugepages=4" in the kernel cmdline.)
The result is got with my 16G x86_64 desktop:

   -------------------------------------------------
	counters.sh (1024M: 32):        FAIL mmap failed: Cannot allocate memory
	counters.sh (1024M: 64):        PASS
	********** TEST SUMMARY
	*                      1024M         
	*                      32-bit 64-bit 
	*     Total testcases:     1      1   
	*             Skipped:     0      0   
	*                PASS:     0      1   
	*                FAIL:     1      0   
	*    Killed by signal:     0      0   
	*   Bad configuration:     0      0   
	*       Expected FAIL:     0      0   
	*     Unexpected PASS:     0      0   
	*    Test not present:     0      0   
	* Strange test result:     0      0   
	**********
   -------------------------------------------------

The test passes for 64bit, but fails for 32bit (but I think it's okay,
since 1G hugetlb page is too large for the 32bit).				 

> giga pages. In other words surplus pages and their accounting is not
> supported at all.
Yes.

> 
> I haven't yet checked your patchset but I can tell you one thing.
Could you please review the patch set when you have time? Thanks a lot.

> Surplus and subpool pages code is tricky as hell. And it is not just a
Agree. 

Do we really need so many accountings? such as reserve/ovorcommit/surplus.

> matter of teaching the huge page allocation code to do the right thing.
> There are subtle details all over the place. E.g. we currently
> do not free giga pages AFAICS. In fact I believe that the giga pages are
Please correct me if I am wrong. :)

I think the free-giga-pages can work well.
Please see the code in update_and_free_page(). 

Could you please list all the subtle details you think the code is wrong?
I can check them one by one.


> kind of implanted to the existing code without any higher level
> consistency. This should change long term. But I am worried it is much
What's type of the "higher level consistency" we should care about?

Thanks
Huang Shijie
> more work.
> 
> Now I might be wrong because I might misremember things which might have
> been changed recently but please make sure you describe the current
> state and changes of giga pages when touching this area much better if
> you want to pursue this route...
> 

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