Re: [PATCH] mm: Move __vma_address() to internal.h to be inlined in huge_memory.c

[Waiman Long]

On 06/12/2014 05:45 PM, David Rientjes wrote:
> On Thu, 12 Jun 2014, Waiman Long wrote:
>
> > > > The vma_address() function which is used to compute the virtual address
> > > > within a VMA is used only by 2 files in the mm subsystem - rmap.c and
> > > > huge_memory.c. This function is defined in rmap.c and is inlined by
> > > > its callers there, but it is also declared as an external function.
> > > >
> > > > However, the __split_huge_page() function which calls vma_address()
> > > > in huge_memory.c is calling it as a real function call. This is not
> > > > as efficient as an inlined function. This patch moves the underlying
> > > > inlined __vma_address() function to internal.h to be shared by both
> > > > the rmap.c and huge_memory.c file.
> > > This increases huge_memory.o's text+data_bss by 311 bytes, which makes
> > > me suspect that it is a bad change due to its increase of kernel cache
> > > footprint.
> > >
> > > Perhaps we should be noinlining __vma_address()?
> > On my test machine, I saw an increase of 144 bytes in the text segment
> > of huge_memory.o. The size in size is caused by an increase in the size
> > of the __split_huge_page function. When I remove the
> >
> >          if (unlikely(is_vm_hugetlb_page(vma)))
> >                  pgoff = page->index<<  huge_page_order(page_hstate(page));
> >
> > check, the increase in size drops down to 24 bytes. As a THP cannot be
> > a hugetlb page, there is no point in doing this check for a THP. I will
> > update the patch to pass in an additional argument to disable this
> > check for __split_huge_page.
> I think we're seeking a reason or performance numbers that suggest
> __vma_address() being inline is appropriate and so far we lack any such
> evidence.  Adding additional parameters to determine checks isn't going to
> change the fact that it increases text size needlessly.

This patch was motivated by my investigation of a freeze problem of an 
application running on SLES11 sp3 which was caused by the long time it 
took to munmap part of a THP. Inlining vma_address help a bit in that 
situation. However, the problem will be essentially gone after including 
patches that changing the anon_vma_chain to use rbtree instead of a 
simple list.

I do agree that performance impact of inlining vma_address in minimal in 
the latest kernel. So I am not going to pursue this any further.

Thank for the review.

-Longman
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