Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

["Zi Yan" <zi.yan@cs.rutgers.edu>]

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On 25 Oct 2018, at 4:10, Anshuman Khandual wrote:

> On 10/16/2018 08:01 PM, Zi Yan wrote:
>> On 15 Oct 2018, at 0:06, Anshuman Khandual wrote:
>>
>>> On 10/15/2018 06:23 AM, Zi Yan wrote:
>>>> On 12 Oct 2018, at 4:00, Anshuman Khandual wrote:
>>>>
>>>>> On 10/10/2018 06:13 PM, Zi Yan wrote:
>>>>>> On 10 Oct 2018, at 0:05, Anshuman Khandual wrote:
>>>>>>
>>>>>>> On 10/09/2018 07:28 PM, Zi Yan wrote:
>>>>>>>> cc: Naoya Horiguchi (who proposed to use !_PAGE_PRESENT && !_PAGE_PSE for x86
>>>>>>>> PMD migration entry check)
>>>>>>>>
>>>>>>>> On 8 Oct 2018, at 23:58, 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()
>>>>>>>>
>>>>>>>> !pmd_present() && pmd_trans_huge() is used to represent THPs under splitting,
>>>>>>>
>>>>>>> Not really if we just look at code in the conditional blocks.
>>>>>>
>>>>>> Yeah, I explained it wrong above. Sorry about that.
>>>>>>
>>>>>> In x86, pmd_present() checks (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE),
>>>>>> thus, it returns true even if the present bit is cleared but PSE bit is set.
>>>>>
>>>>> Okay.
>>>>>
>>>>>> This is done so, because THPs under splitting are regarded as present in the kernel
>>>>>> but not present when a hardware page table walker checks it.
>>>>>
>>>>> Okay.
>>>>>
>>>>>>
>>>>>> For PMD migration entry, which should be regarded as not present, if PSE bit
>>>>>> is set, which makes pmd_trans_huge() returns true, like ARM64 does, all
>>>>>> PMD migration entries will be regarded as present
>>>>>
>>>>> Okay to make pmd_present() return false pmd_trans_huge() has to return false
>>>>> as well. Is there anything which can be done to get around this problem on
>>>>> X86 ? pmd_trans_huge() returning true for a migration entry sounds logical.
>>>>> Otherwise we would revert the condition block order to accommodate both the
>>>>> implementation for pmd_trans_huge() as suggested by Kirill before or just
>>>>> consider this patch forward.
>>>>>
>>>>> Because I am not really sure yet about the idea of getting pmd_present()
>>>>> check into pmd_trans_huge() on arm64 just to make it fit into this semantics
>>>>> as suggested by Will. If a PMD is trans huge page or not should not depend on
>>>>> whether it is present or not.
>>>>
>>>> In terms of THPs, we have three cases: a present THP, a THP under splitting,
>>>> and a THP under migration. pmd_present() and pmd_trans_huge() both return true
>>>> for a present THP and a THP under splitting, because they discover _PAGE_PSE bit
>>>
>>> Then how do we differentiate between a mapped THP and a splitting THP.
>>
>> AFAIK, in x86, there is no distinction between a mapped THP and a splitting THP
>> using helper functions.
>>
>> A mapped THP has _PAGE_PRESENT bit and _PAGE_PSE bit set, whereas a splitting THP
>> has only _PAGE_PSE bit set. But both pmd_present() and pmd_trans_huge() return
>> true as long as _PAGE_PSE bit is set.
>
> I understand that. What I was wondering was since there is a need to differentiate
> between a mapped THP and a splitting THP at various places in generic THP, we would
> need to way to identify each of them unambiguously some how. Is that particular
> assumption wrong ? Dont we need to differentiate between a mapped THP and THP under
> splitting ?

According to Andrea's explanation here: https://lore.kernel.org/patchwork/patch/997412/#1184298,
we do not distinguish between a mapped THP and a splitting THP, because pmd_to_page()
can return valid pages for both cases.

>>
>>>
>>>> is set for both cases, whereas they both return false for a THP under migration.
>>>> You want to change them to make pmd_trans_huge() returns true for a THP under migration
>>>> instead of false to help ARM64’s support for THP migration.
>>> I am just trying to understand the rationale behind this semantics and see where
>>> it should be fixed.
>>>
>>> I think the fundamental problem here is that THP under split has been difficult
>>> to be re-presented through the available helper functions and in turn PTE bits.
>>>
>>> The following checks
>>>
>>> 1) pmd_present()
>>> 2) pmd_trans_huge()
>>>
>>> Represent three THP states
>>>
>>> 1) Mapped THP		(pmd_present && pmd_trans_huge)
>>> 2) Splitting THP	(pmd_present && pmd_trans_huge)
>>> 3) Migrating THP	(!pmd_present && !pmd_trans_huge)
>>>
>>> The problem is if we make pmd_trans_huge() return true for all the three states
>>> which sounds logical because they are all still trans huge PMD, then pmd_present()
>>> can only represent two states not three as required.
>>
>> We are on the same page about representing three THP states in x86.
>> I also agree with you that it is logical to use three distinct representations
>> for these three states, i.e. splitting THP could be changed to (!pmd_present && pmd_trans_huge
>
> Right. Also we need clear wrapper around them in line with is_pmd_migration_entry() to
> represent three states all of which calling pmd_present() and pmd_trans_huge() which
> are exported by various architectures with exact same semantics without any ambiguity.
>
> 1) is_pmd_mapped_entry()
> 2) is_pmd_splitting_entry()
> 3) is_pmd_migration_entry()

I think the semantics of pmd_trans_huge() is that the pmd entry is pointing to
a huge page. So is_pmd_mapped_entry() is the same as is_pmd_splitting_entry()
in terms of that.

According to Andrea's explanation:https://lore.kernel.org/patchwork/patch/997412/#1184298,
the semantics can avoid pmd_lock serializations on all VM fast paths, which
is valid IMHO.


>>
>>
>>>>
>>>> For x86, this change requires:
>>>> 1. changing the condition in pmd_trans_huge(), so that it returns true for
>>>> PMD migration entries;
>>>> 2. changing the code, which calls pmd_trans_huge(), to match the new logic.
>>> Can those be fixed with an additional check for pmd_present() as suggested here
>>> in this patch ? Asking because in case we could not get common semantics for
>>> these helpers on all arch that would be a fall back option for the moment.
>>
>> It would be OK for x86, since pmd_trans_huge() implies pmd_present() and hence
>> adding pmd_present() to pmd_trans_huge() makes no difference. But for ARM64,
>> from my understanding of the code described below, adding pmd_present() to
>> pmd_trans_huge() seems to exclude splitting THPs from the original semantic.
>>
>>
>>>>
>>>> Another problem I see is that x86’s pmd_present() returns true for a THP under
>>>> splitting but ARM64’s pmd_present() returns false for a THP under splitting.
>>>
>>> But how did you conclude this ? I dont see any explicit helper for splitting
>>> THP. Could you please point me in the code ?
>>
>> From the code I read for ARM64
>> (https://elixir.bootlin.com/linux/v4.19-rc8/source/arch/arm64/include/asm/pgtable.h#L360
>> and https://elixir.bootlin.com/linux/v4.19-rc8/source/arch/arm64/include/asm/pgtable.h#L86),
>> pmd_present() only checks _PAGE_PRESENT and _PAGE_PROTONE. During a THP splitting,
>
> These are PTE_VALID and PTE_PROT_NONE instead on arm64. But yes, they are equivalent
> to __PAGE_PRESENT and __PAGE_PROTNONE on other archs.
>
> #define pmd_present(pmd)        pte_present(pmd_pte(pmd))
> #define pte_present(pte)        (!!(pte_val(pte) & (PTE_VALID | PTE_PROT_NONE)))
>
>> pmdp_invalidate() clears _PAGE_PRESENT (https://elixir.bootlin.com/linux/v4.19-rc8/source/mm/huge_memory.c#L2130). So pmd_present() returns false in ARM64. Let me know
>> if I got anything wrong.
>>
>
> old_pmd = pmdp_invalidate(vma, haddr, pmd);
>
> __split_huge_pmd_locked -> pmdp_invalidate (the above mentioned instance)
> pmdp_invalidate -> pmd_mknotpresent
>
> #define pmd_mknotpresent(pmd)   (__pmd(pmd_val(pmd) & ~PMD_SECT_VALID)
>
> Generic pmdp invalidation removes PMD_SECT_VALID from a mapped PMD entry.
> PMD_SECT_VALID is similar to PTE_VALID through identified separately. So you
> are right, on arm64 pmd_present() return false for THP under splitting.

This may actually cause problems in arm64, since the kernel will miss all splitting THPs.

In sum, according to Andrea's explanation, I think it is better to adjust
arm64's pmd_present() and pmd_trans_huge() to match what x86's semantics.
Otherwise, arm64 might hit bugs while handling THPs.

--
Best Regards
Yan Zi

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