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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Naoya Horiguchi wrote:
> 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.

No, before the patch mm->page_table_lock is what we need: it serializes
setting up pmd, not adding pte to pmd and it's subject to change with new
locking model.

-- 
 Kirill A. Shutemov

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