Re: [PATCH 2/2] thp: support split page table lock

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

Hi Kirill,

On Fri, Sep 06, 2013 at 01:48:03PM +0300, Kirill A. Shutemov wrote:
> Naoya Horiguchi wrote:
> > Thp related code also uses per process mm->page_table_lock now.
> > So making it fine-grained can provide better performance.
> > 
> > This patch makes thp support split page table lock by using page->ptl
> > of the pages storing "pmd_trans_huge" pmds.
> > 
> > Some functions like pmd_trans_huge_lock() and page_check_address_pmd()
> > are expected by their caller to pass back the pointer of ptl, so this
> > patch adds to those functions new arguments for that. Rather than that,
> > this patch gives only straightforward replacement.
> > 
> > ChangeLog v3:
> >  - fixed argument of huge_pmd_lockptr() in copy_huge_pmd()
> >  - added missing declaration of ptl in do_huge_pmd_anonymous_page()
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> 
> Generally, looks good. Few notes:
> 
> I believe you need to convert __pte_alloc() to new locking. Not sure about
> __pte_alloc_kernel().
> Have you check all rest mm->page_table_lock, that they shouldn't be
> converted to new locking?

I thought that keeping __pte_alloc() using mm->page_table_lock was safe
because it uses bare mm->page_table_lock instead of pte_lockptr() even
before this patchset, but not 100% sure.
__pte_alloc() (and its family) are used in normal page path, so if it's
not safe, we've lived with unsafe code for very long (maybe since 2005).
Anyway, converting __pte_alloc() into split ptl could improve performance
(though we need testing to know what amount), so I'll try that.

> You use uninitialized_var() a lot. It's ugly. I've check few places
> (task_mmu.c, copy_huge_pmd) and have found a reason why we need it there.
> Why?

I got a compile warning of uninitialized usage when developing and added
to suppress it, but in the final form I never get such a warning.
So I'll remove this uninitialized_var()s.

> You often do
> 
> +       ptl = huge_pmd_lockptr(mm, pmd);
> +       spin_lock(ptl);
> 
> Should we have a helper to combine them? huge_pmd_lock()?

OK, I'll do it.

Thanks,
Naoya Horiguchi

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