* [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
@ 2021-12-17 2:34 Li Xinhai
2021-12-17 3:01 ` Huang, Ying
0 siblings, 1 reply; 3+ messages in thread
From: Li Xinhai @ 2021-12-17 2:34 UTC (permalink / raw)
To: linux-mm; +Cc: akpm, Zi Yan, Huang, Ying, Kirill A. Shutemov
When BUG_ON check for THP migration entry, the exsiting code only check
thp_migration_supported case, but not for !thp_migration_supported case.
If !thp_migration_supported() and !pmd_present(), the original code may
dead loop in theory. To make the BUG_ON check consistent, we need catch
both cases.
Move the BUG_ON check one step eariler, because if the bug happen we
should know it instead of depend on FOLL_MIGRATION been used by caller.
Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
check, the existing code don't help to avoid useless locking within
pmd_migration_entry_wait(), so remove that check.
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
---
V2->V3:
mention about the dead loop in commit message.
V1->V2:
Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
for it.
mm/gup.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 2c51e9748a6a..94d0e586ca0b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
}
retry:
if (!pmd_present(pmdval)) {
+ /*
+ * Should never reach here, if thp migration is not supported;
+ * Otherwise, it must be a thp miration entry.
+ */
+ VM_BUG_ON(!thp_migration_supported() ||
+ !is_pmd_migration_entry(pmdval));
+
if (likely(!(flags & FOLL_MIGRATION)))
return no_page_table(vma, flags);
- VM_BUG_ON(thp_migration_supported() &&
- !is_pmd_migration_entry(pmdval));
- if (is_pmd_migration_entry(pmdval))
- pmd_migration_entry_wait(mm, pmd);
+
+ pmd_migration_entry_wait(mm, pmd);
pmdval = READ_ONCE(*pmd);
/*
* MADV_DONTNEED may convert the pmd to null because
--
2.27.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
2021-12-17 2:34 [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask Li Xinhai
@ 2021-12-17 3:01 ` Huang, Ying
2021-12-17 5:49 ` Li Xinhai
0 siblings, 1 reply; 3+ messages in thread
From: Huang, Ying @ 2021-12-17 3:01 UTC (permalink / raw)
To: Li Xinhai; +Cc: linux-mm, akpm, Zi Yan, Kirill A. Shutemov
Li Xinhai <lixinhai.lxh@gmail.com> writes:
> When BUG_ON check for THP migration entry, the exsiting code only check
s/exsiting/existing/
Found some misspelling in the comments too. Please fix them with some
tool.
Best Regards,
Huang, Ying
> thp_migration_supported case, but not for !thp_migration_supported case.
> If !thp_migration_supported() and !pmd_present(), the original code may
> dead loop in theory. To make the BUG_ON check consistent, we need catch
> both cases.
>
> Move the BUG_ON check one step eariler, because if the bug happen we
> should know it instead of depend on FOLL_MIGRATION been used by caller.
>
> Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
> check, the existing code don't help to avoid useless locking within
> pmd_migration_entry_wait(), so remove that check.
>
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> ---
> V2->V3:
> mention about the dead loop in commit message.
>
> V1->V2:
> Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
> for it.
>
>
> mm/gup.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..94d0e586ca0b 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> }
> retry:
> if (!pmd_present(pmdval)) {
> + /*
> + * Should never reach here, if thp migration is not supported;
> + * Otherwise, it must be a thp miration entry.
> + */
> + VM_BUG_ON(!thp_migration_supported() ||
> + !is_pmd_migration_entry(pmdval));
> +
> if (likely(!(flags & FOLL_MIGRATION)))
> return no_page_table(vma, flags);
> - VM_BUG_ON(thp_migration_supported() &&
> - !is_pmd_migration_entry(pmdval));
> - if (is_pmd_migration_entry(pmdval))
> - pmd_migration_entry_wait(mm, pmd);
> +
> + pmd_migration_entry_wait(mm, pmd);
> pmdval = READ_ONCE(*pmd);
> /*
> * MADV_DONTNEED may convert the pmd to null because
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
2021-12-17 3:01 ` Huang, Ying
@ 2021-12-17 5:49 ` Li Xinhai
0 siblings, 0 replies; 3+ messages in thread
From: Li Xinhai @ 2021-12-17 5:49 UTC (permalink / raw)
To: Huang, Ying; +Cc: linux-mm, akpm, Zi Yan, Kirill A. Shutemov
On 12/17/21 11:01 AM, Huang, Ying wrote:
> Li Xinhai <lixinhai.lxh@gmail.com> writes:
>
>> When BUG_ON check for THP migration entry, the exsiting code only check
>
> s/exsiting/existing/
>
> Found some misspelling in the comments too. Please fix them with some
> tool.
>
Yes, those need be fixed, thanks.
> Best Regards,
> Huang, Ying
>
>> thp_migration_supported case, but not for !thp_migration_supported case.
>> If !thp_migration_supported() and !pmd_present(), the original code may
>> dead loop in theory. To make the BUG_ON check consistent, we need catch
>> both cases.
>>
>> Move the BUG_ON check one step eariler, because if the bug happen we
>> should know it instead of depend on FOLL_MIGRATION been used by caller.
>>
>> Because pmdval instead of *pmd is read by the is_pmd_migration_entry()
>> check, the existing code don't help to avoid useless locking within
>> pmd_migration_entry_wait(), so remove that check.
>>
>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: "Huang, Ying" <ying.huang@intel.com>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>> ---
>> V2->V3:
>> mention about the dead loop in commit message.
>>
>> V1->V2:
>> Move the BUG_ON() check before if(!(flags & FOLL_MIGRATION)); and add comments
>> for it.
>>
>>
>> mm/gup.c | 13 +++++++++----
>> 1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 2c51e9748a6a..94d0e586ca0b 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -642,12 +642,17 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>> }
>> retry:
>> if (!pmd_present(pmdval)) {
>> + /*
>> + * Should never reach here, if thp migration is not supported;
>> + * Otherwise, it must be a thp miration entry.
>> + */
>> + VM_BUG_ON(!thp_migration_supported() ||
>> + !is_pmd_migration_entry(pmdval));
>> +
>> if (likely(!(flags & FOLL_MIGRATION)))
>> return no_page_table(vma, flags);
>> - VM_BUG_ON(thp_migration_supported() &&
>> - !is_pmd_migration_entry(pmdval));
>> - if (is_pmd_migration_entry(pmdval))
>> - pmd_migration_entry_wait(mm, pmd);
>> +
>> + pmd_migration_entry_wait(mm, pmd);
>> pmdval = READ_ONCE(*pmd);
>> /*
>> * MADV_DONTNEED may convert the pmd to null because
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-12-17 2:34 [PATCH V3] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask Li Xinhai
2021-12-17 3:01 ` Huang, Ying
2021-12-17 5:49 ` Li Xinhai
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