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

[Anshuman Khandual <anshuman.khandual@arm.com>]

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.

> 
> My concern is that if ARM64’s pmd_trans_huge() returns true for migration
> entries, unlike x86, there might be bugs triggered in the kernel when
> THP migration is enabled in ARM64.

Right and that is exactly what we are trying to fix with this patch.

>
> Let me know if I explain this clear to you.
> 
>>
>>> since _PAGE_PRESENT is cleared during THP splitting but _PAGE_PSE is not.
>>> See the comment in pmd_present() for x86, in arch/x86/include/asm/pgtable.h
>>
>>
>>         if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
>>                 pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>                 if (likely(pmd_trans_huge(*pvmw->pmd))) {
>>                         if (pvmw->flags & PVMW_MIGRATION)
>>                                 return not_found(pvmw);
>>                         if (pmd_page(*pvmw->pmd) != page)
>>                                 return not_found(pvmw);
>>                         return true;
>>                 } else if (!pmd_present(*pvmw->pmd)) {
>>                         if (thp_migration_supported()) {
>>                                 if (!(pvmw->flags & PVMW_MIGRATION))
>>                                         return not_found(pvmw);
>>                                 if (is_migration_entry(pmd_to_swp_entry(*pvmw->pmd))) {
>>                                         swp_entry_t entry = pmd_to_swp_entry(*pvmw->pmd);
>>
>>                                         if (migration_entry_to_page(entry) != page)
>>                                                 return not_found(pvmw);
>>                                         return true;
>>                                 }
>>                         }
>>                         return not_found(pvmw);
>>                 } else {
>>                         /* THP pmd was split under us: handle on pte level */
>>                         spin_unlock(pvmw->ptl);
>>                         pvmw->ptl = NULL;
>>                 }
>>         } else if (!pmd_present(pmde)) { ---> Outer 'else if'
>>                 return false;
>>         }
>>
>> Looking at the above code, it seems the conditional check for a THP
>> splitting case would be (!pmd_trans_huge && pmd_present) instead as
>> it has skipped the first two conditions. But THP splitting must have
>> been initiated once it has cleared the outer check (else it would not
>> have cleared otherwise)
>>
>> if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)).
> 
> If a THP is under splitting, both pmd_present() and pmd_trans_huge() return
> true in x86. The else part (/* THP pmd was split under us … */) happens
> after splitting is done.

Okay, got it.

> 
>> BTW what PMD state does the outer 'else if' block identify which must
>> have cleared the following condition to get there.
>>
>> (!pmd_present && !pmd_trans_huge && !is_pmd_migration_entry)
> 
> I think it is the case that the PMD is gone or equivalently pmd_none().
> This PMD entry is not in use.

Okay, got it.