Re: [PATCHv2 4/4] mm: make compound_head() robust

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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Johannes Weiner <hannes@cmpxchg.org>,
	David Rientjes <rientjes@google.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv2 4/4] mm: make compound_head() robust
Date: Tue, 18 Aug 2015 21:24:41 +0300	[thread overview]
Message-ID: <20150818182441.GC21383@node.dhcp.inet.fi> (raw)
In-Reply-To: <20150818164112.GN5033@dhcp22.suse.cz>

On Tue, Aug 18, 2015 at 06:41:13PM +0200, Michal Hocko wrote:
> On Tue 18-08-15 13:20:22, Michal Hocko wrote:
> > On Mon 17-08-15 18:09:05, Kirill A. Shutemov wrote:
> > > Hugh has pointed that compound_head() call can be unsafe in some
> > > context. There's one example:
> > > 
> > > 	CPU0					CPU1
> > > 
> > > isolate_migratepages_block()
> > >   page_count()
> > >     compound_head()
> > >       !!PageTail() == true
> > > 					put_page()
> > > 					  tail->first_page = NULL
> > >       head = tail->first_page
> > > 					alloc_pages(__GFP_COMP)
> > > 					   prep_compound_page()
> > > 					     tail->first_page = head
> > > 					     __SetPageTail(p);
> > >       !!PageTail() == true
> > >     <head == NULL dereferencing>
> > > 
> > > The race is pure theoretical. I don't it's possible to trigger it in
> > > practice. But who knows.
> > > 
> > > We can fix the race by changing how encode PageTail() and compound_head()
> > > within struct page to be able to update them in one shot.
> > > 
> > > The patch introduces page->compound_head into third double word block in
> > > front of compound_dtor and compound_order. That means it shares storage
> > > space with:
> > > 
> > >  - page->lru.next;
> > >  - page->next;
> > >  - page->rcu_head.next;
> > >  - page->pmd_huge_pte;
> > > 
> > > That's too long list to be absolutely sure, but looks like nobody uses
> > > bit 0 of the word. It can be used to encode PageTail(). And if the bit
> > > set, rest of the word is pointer to head page.
> > 
> > I didn't look too closely but the general idea makes sense to me and the
> > overal code simplification is sound. I will give it more detailed review
> > after I sort out other stuff.
> 
> AFICS page::first_page wasn't used outside of compound page logic so you
> should remove it in this patch. The rest looks good to me.

I missed it by accident during rework for v2. Will fix.

> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

-- 
 Kirill A. Shutemov

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WARNING: multiple messages have this Message-ID (diff)

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Johannes Weiner <hannes@cmpxchg.org>,
	David Rientjes <rientjes@google.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv2 4/4] mm: make compound_head() robust
Date: Tue, 18 Aug 2015 21:24:41 +0300	[thread overview]
Message-ID: <20150818182441.GC21383@node.dhcp.inet.fi> (raw)
In-Reply-To: <20150818164112.GN5033@dhcp22.suse.cz>

On Tue, Aug 18, 2015 at 06:41:13PM +0200, Michal Hocko wrote:
> On Tue 18-08-15 13:20:22, Michal Hocko wrote:
> > On Mon 17-08-15 18:09:05, Kirill A. Shutemov wrote:
> > > Hugh has pointed that compound_head() call can be unsafe in some
> > > context. There's one example:
> > > 
> > > 	CPU0					CPU1
> > > 
> > > isolate_migratepages_block()
> > >   page_count()
> > >     compound_head()
> > >       !!PageTail() == true
> > > 					put_page()
> > > 					  tail->first_page = NULL
> > >       head = tail->first_page
> > > 					alloc_pages(__GFP_COMP)
> > > 					   prep_compound_page()
> > > 					     tail->first_page = head
> > > 					     __SetPageTail(p);
> > >       !!PageTail() == true
> > >     <head == NULL dereferencing>
> > > 
> > > The race is pure theoretical. I don't it's possible to trigger it in
> > > practice. But who knows.
> > > 
> > > We can fix the race by changing how encode PageTail() and compound_head()
> > > within struct page to be able to update them in one shot.
> > > 
> > > The patch introduces page->compound_head into third double word block in
> > > front of compound_dtor and compound_order. That means it shares storage
> > > space with:
> > > 
> > >  - page->lru.next;
> > >  - page->next;
> > >  - page->rcu_head.next;
> > >  - page->pmd_huge_pte;
> > > 
> > > That's too long list to be absolutely sure, but looks like nobody uses
> > > bit 0 of the word. It can be used to encode PageTail(). And if the bit
> > > set, rest of the word is pointer to head page.
> > 
> > I didn't look too closely but the general idea makes sense to me and the
> > overal code simplification is sound. I will give it more detailed review
> > after I sort out other stuff.
> 
> AFICS page::first_page wasn't used outside of compound page logic so you
> should remove it in this patch. The rest looks good to me.

I missed it by accident during rework for v2. Will fix.

> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

-- 
 Kirill A. Shutemov

next prev parent reply	other threads:[~2015-08-18 18:24 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-08-17 15:09 [PATCHv2 0/4] Fix compound_head() race Kirill A. Shutemov
2015-08-17 15:09 ` Kirill A. Shutemov
2015-08-17 15:09 ` [PATCHv2 1/4] mm: drop page->slab_page Kirill A. Shutemov
2015-08-17 15:09   ` Kirill A. Shutemov
2015-08-18  0:43   ` David Rientjes
2015-08-18  0:43     ` David Rientjes
2015-08-17 15:09 ` [PATCHv2 2/4] zsmalloc: use page->private instead of page->first_page Kirill A. Shutemov
2015-08-17 15:09   ` Kirill A. Shutemov
2015-08-25  1:49   ` Sergey Senozhatsky
2015-08-25  1:49     ` Sergey Senozhatsky
2015-08-25  2:17   ` Sergey Senozhatsky
2015-08-25  2:17     ` Sergey Senozhatsky
2015-08-25 17:10     ` Kirill A. Shutemov
2015-08-25 17:10       ` Kirill A. Shutemov
2015-08-17 15:09 ` [PATCHv2 3/4] mm: pack compound_dtor and compound_order into one word in struct page Kirill A. Shutemov
2015-08-17 15:09   ` Kirill A. Shutemov
2015-08-17 22:59   ` Hugh Dickins
2015-08-17 22:59     ` Hugh Dickins
2015-08-18 15:43   ` Michal Hocko
2015-08-18 15:43     ` Michal Hocko
2015-08-18 18:22     ` Kirill A. Shutemov
2015-08-18 18:22       ` Kirill A. Shutemov
2015-08-18 16:05   ` Michal Hocko
2015-08-18 16:05     ` Michal Hocko
2015-08-18 18:22     ` Kirill A. Shutemov
2015-08-18 18:22       ` Kirill A. Shutemov
2015-08-17 15:09 ` [PATCHv2 4/4] mm: make compound_head() robust Kirill A. Shutemov
2015-08-17 15:09   ` Kirill A. Shutemov
2015-08-18 11:20   ` Michal Hocko
2015-08-18 11:20     ` Michal Hocko
2015-08-18 16:41     ` Michal Hocko
2015-08-18 16:41       ` Michal Hocko
2015-08-18 18:24       ` Kirill A. Shutemov [this message]
2015-08-18 18:24         ` Kirill A. Shutemov

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