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From: "Aneesh Kumar K.V" <aneeshkumar.opensource@gmail.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kirill.shutemov@linux.intel.com, akpm@linux-foundation.org,
	mhocko@suse.com, zi.yan@cs.rutgers.edu
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry
Date: Mon, 15 Oct 2018 14:02:48 +0530	[thread overview]
Message-ID: <4bf3951d-410f-fac4-dfb2-7dee5568e6ff@linux.ibm.com> (raw)
In-Reply-To: <fb0ee5dd-5799-f5af-891a-992dd9a16a9f@arm.com>

On 10/12/18 1:32 PM, Anshuman Khandual wrote:
> 
> 
> On 10/09/2018 06:48 PM, Will Deacon wrote:
>> On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote:
>>> On Tue, Oct 09, 2018 at 09:28:58AM +0530, Anshuman Khandual wrote:
>>>> A normal mapped THP page at PMD level should be correctly differentiated
>>>> from a PMD migration entry while walking the page table. A mapped THP would
>>>> additionally check positive for pmd_present() along with pmd_trans_huge()
>>>> as compared to a PMD migration entry. This just adds a new conditional test
>>>> differentiating the two while walking the page table.
>>>>
>>>> Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path")
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>> ---
>>>> On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually
>>>> exclusive which makes the current conditional block work for both mapped
>>>> and migration entries. This is not same with arm64 where pmd_trans_huge()
>>>> returns positive for both mapped and migration entries. Could some one
>>>> please explain why pmd_trans_huge() has to return false for migration
>>>> entries which just install swap bits and its still a PMD ?
>>>
>>> I guess it's just a design choice. Any reason why arm64 cannot do the
>>> same?
>>
>> Anshuman, would it work to:
>>
>> #define pmd_trans_huge(pmd)     (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
> yeah this works but some how does not seem like the right thing to do
> but can be the very last option.
> 


There can be other code paths that makes that assumption. I ended up 
doing the below for pmd_trans_huge on ppc64.

/*
  * Only returns true for a THP. False for pmd migration entry.
  * We also need to return true when we come across a pte that
  * in between a thp split. While splitting THP, we mark the pmd
  * invalid (pmdp_invalidate()) before we set it with pte page
  * address. A pmd_trans_huge() check against a pmd entry during that time
  * should return true.
  * We should not call this on a hugetlb entry. We should check for HugeTLB
  * entry using vma->vm_flags
  * The page table walk rule is explained in Documentation/vm/transhuge.rst
  */
static inline int pmd_trans_huge(pmd_t pmd)
{
	if (!pmd_present(pmd))
		return false;

	if (radix_enabled())
		return radix__pmd_trans_huge(pmd);
	return hash__pmd_trans_huge(pmd);
}

-aneesh

  reply	other threads:[~2018-10-15  8:32 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-09  3:58 Anshuman Khandual
2018-10-09 13:04 ` Kirill A. Shutemov
2018-10-09 13:18   ` Will Deacon
2018-10-12  8:02     ` Anshuman Khandual
2018-10-15  8:32       ` Aneesh Kumar K.V [this message]
2018-10-16 13:16         ` Anshuman Khandual
2018-10-09 13:42   ` Anshuman Khandual
2018-10-09 13:58 ` Zi Yan
2018-10-10  4:05   ` Anshuman Khandual
2018-10-10 12:43     ` Zi Yan
2018-10-12  8:00       ` Anshuman Khandual
2018-10-15  0:53         ` Zi Yan
2018-10-15  4:06           ` Anshuman Khandual
2018-10-16 14:31             ` Zi Yan
2018-10-18  2:17               ` Naoya Horiguchi
2018-11-02  5:22                 ` Anshuman Khandual
2018-10-25  8:10               ` Anshuman Khandual
2018-10-25 18:45                 ` Zi Yan
2018-10-26  1:39                   ` Anshuman Khandual
2018-10-17  2:09           ` Andrea Arcangeli
2018-10-22 14:00             ` Zi Yan
2018-11-02  6:15             ` Anshuman Khandual
2018-11-06  0:35               ` Will Deacon
2018-11-06  9:51                 ` Anshuman Khandual

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