linux-kernel - Re: [PATCH -mm v2 04/11] pagewalk: move pmd_trans_huge

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Date:	Wed, 18 Jun 2014 17:13:53 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	linux-mm@...ck.org, Dave Hansen <dave.hansen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 04/11] pagewalk: move pmd_trans_huge_lock() from
 callbacks to common code

On 06/17/2014 05:01 PM, Naoya Horiguchi wrote:
> On Tue, Jun 17, 2014 at 04:27:56PM +0200, Jerome Marchand wrote:
>> On 06/12/2014 11:48 PM, Naoya Horiguchi wrote:
>>> Now all of current users of page table walker are canonicalized, i.e.
>>> pmd_entry() handles only trans_pmd entry, and pte_entry() handles pte entry.
>>> So we can factorize common code more.
>>> This patch moves pmd_trans_huge_lock() in each pmd_entry() to pagewalk core.
>>>
>>> ChangeLog v2:
>>> - add null check walk->vma in walk_pmd_range()
>>
>> An older version of this patch already made it to linux-next (commit
>> b0e08c5) and I've actually hit the NULL pointer dereference.
>>
>> Moreover, that patch (or maybe another recent pagewalk patch) breaks
>> /proc/<pid>/smaps. All fields that should have been filled by
>> smaps_pte() are almost always zero (and when it isn't, it's always a
>> multiple of 2MB). It seems to me that the page walk never goes below
>> pmd level.
> 
> Agreed, I'm now thinking that forcing pte_entry() for every user is not
> good idea, so I'll return to the start point and just will do only the
> necessary changes (i.e. only iron out the vma handling problem for hugepage.)
> 
> Thanks,
> Naoya Horiguchi
> 
>> Jerome
>>
>>> - move comment update into a separate patch
>>>
>>> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>>> ---


>>> diff --git mmotm-2014-05-21-16-57.orig/mm/pagewalk.c mmotm-2014-05-21-16-57/mm/pagewalk.c
>>> index 24311d6f5c20..f1a3417d0b51 100644
>>> --- mmotm-2014-05-21-16-57.orig/mm/pagewalk.c
>>> +++ mmotm-2014-05-21-16-57/mm/pagewalk.c
>>> @@ -73,8 +73,22 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr,
>>>  			continue;
>>>  		}
>>>  
>>> -		if (walk->pmd_entry) {
>>> -			err = walk->pmd_entry(pmd, addr, next, walk);
>>> +		/*
>>> +		 * We don't take compound_lock() here but no race with splitting
>>> +		 * thp happens because:
>>> +		 *  - if pmd_trans_huge_lock() returns 1, the relevant thp is
>>> +		 *    not under splitting, which means there's no concurrent
>>> +		 *    thp split,
>>> +		 *  - if another thread runs into split_huge_page() just after
>>> +		 *    we entered this if-block, the thread must wait for page
>>> +		 *    table lock to be unlocked in __split_huge_page_splitting(),
>>> +		 *    where the main part of thp split is not executed yet.
>>> +		 */
>>> +		if (walk->pmd_entry && walk->vma) {
>>> +			if (pmd_trans_huge_lock(pmd, walk->vma, &walk->ptl) == 1) {
>>> +				err = walk->pmd_entry(pmd, addr, next, walk);
>>> +				spin_unlock(walk->ptl);
>>> +			}
>>>  			if (skip_lower_level_walking(walk))
>>>  				continue;
>>>  			if (err)

This is the cause of the smaps trouble. This code modifies walk->control
when pmd_entry() is present, even when it is not called. All the control
code should depend on pmd_trans_huge_lock() == 1 too.

Jerome
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