linux-kernel - Re: [PATCHv2, RFC 17/30] thp: wait_split_huge_page(): serialize over i_mmap

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Date:	Thu, 28 Mar 2013 17:08:28 +0200 (EET)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Dave <dave@...1.net>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2, RFC 17/30] thp: wait_split_huge_page(): serialize over
 i_mmap_mutex too

Dave wrote:
> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -113,11 +113,20 @@ extern void __split_huge_page_pmd(struct vm_area_struct *vma,
> >  			__split_huge_page_pmd(__vma, __address,		\
> >  					____pmd);			\
> >  	}  while (0)
> > -#define wait_split_huge_page(__anon_vma, __pmd)				\
> > +#define wait_split_huge_page(__vma, __pmd)				\
> >  	do {								\
> >  		pmd_t *____pmd = (__pmd);				\
> > -		anon_vma_lock_write(__anon_vma);			\
> > -		anon_vma_unlock_write(__anon_vma);			\
> > +		struct address_space *__mapping =			\
> > +					vma->vm_file->f_mapping;	\
> > +		struct anon_vma *__anon_vma = (__vma)->anon_vma;	\
> > +		if (__mapping)						\
> > +			mutex_lock(&__mapping->i_mmap_mutex);		\
> > +		if (__anon_vma) {					\
> > +			anon_vma_lock_write(__anon_vma);		\
> > +			anon_vma_unlock_write(__anon_vma);		\
> > +		}							\
> > +		if (__mapping)						\
> > +			mutex_unlock(&__mapping->i_mmap_mutex);		\
> >  		BUG_ON(pmd_trans_splitting(*____pmd) ||			\
> >  		       pmd_trans_huge(*____pmd));			\
> >  	} while (0)
> 
> That thing was pretty ugly _before_. :)  Any chance this can get turned
> in to a function?

Cyclic headers dependencies... :(

> What's the deal with the i_mmap_mutex operation getting split up?  I'm
> blanking on what kind of pages would have both anon_vmas and a valid
> mapping.

anon_vma_lock protects all anon pages on vma against splitting.
i_mmap_mutex protects shared pages. None pages can be on both sides, but
MAP_PRIVATE file vma can have both anon and shared pages.

As an option we can lookup for struct page with pmd_page(), check
PageAnon() and serialize only relevant lock, but...

Original macro, in fact, guarantees that *all* pages on the vma is not
splitting, not only the pages pmd is poinging on. PageAnon() check will
change semantics a bit. It shouldn't be a problem, but who knows.

Do you want me to add the check?

-- 
 Kirill A. Shutemov
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