* [PATCH] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
@ 2021-12-08 13:39 Li Xinhai
2021-12-08 23:35 ` Andrew Morton
0 siblings, 1 reply; 3+ messages in thread
From: Li Xinhai @ 2021-12-08 13:39 UTC (permalink / raw)
To: linux-mm; +Cc: akpm
When BUG_ON check for THP migration entry, we need also consider
!thp_migration_supported() case, becaouse it is invalid at the code
context. Now, make this check been covered without adding extra cost.
Because pmdval instead of *pmd is read, so just call
pmd_migration_entry_wait().
Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
---
mm/gup.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index 2c51e9748a6a..49f490bae030 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
if (!pmd_present(pmdval)) {
if (likely(!(flags & FOLL_MIGRATION)))
return no_page_table(vma, flags);
- VM_BUG_ON(thp_migration_supported() &&
+ 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] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
2021-12-08 13:39 [PATCH] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask Li Xinhai
@ 2021-12-08 23:35 ` Andrew Morton
2021-12-09 1:40 ` Li Xinhai
0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2021-12-08 23:35 UTC (permalink / raw)
To: Li Xinhai; +Cc: linux-mm
On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@gmail.com> wrote:
> When BUG_ON check for THP migration entry, we need also consider
> !thp_migration_supported() case, becaouse it is invalid at the code
> context. Now, make this check been covered without adding extra cost.
>
> Because pmdval instead of *pmd is read, so just call
> pmd_migration_entry_wait().
>
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
> mm/gup.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2c51e9748a6a..49f490bae030 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> if (!pmd_present(pmdval)) {
> if (likely(!(flags & FOLL_MIGRATION)))
> return no_page_table(vma, flags);
> - VM_BUG_ON(thp_migration_supported() &&
> + VM_BUG_ON(!thp_migration_supported() ||
I don't get this. thp_migration_supported() evaluates to a
compile-time constant. What is the point in doing a runtime BUG based
on a thing which is known at compile-time?
> !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] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask
2021-12-08 23:35 ` Andrew Morton
@ 2021-12-09 1:40 ` Li Xinhai
0 siblings, 0 replies; 3+ messages in thread
From: Li Xinhai @ 2021-12-09 1:40 UTC (permalink / raw)
To: Andrew Morton; +Cc: linux-mm
On 12/9/21 7:35 AM, Andrew Morton wrote:
> On Wed, 8 Dec 2021 21:39:30 +0800 Li Xinhai <lixinhai.lxh@gmail.com> wrote:
>
>> When BUG_ON check for THP migration entry, we need also consider
>> !thp_migration_supported() case, becaouse it is invalid at the code
>> context. Now, make this check been covered without adding extra cost.
>>
>> Because pmdval instead of *pmd is read, so just call
>> pmd_migration_entry_wait().
>>
>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>> ---
>> mm/gup.c | 5 ++---
>> 1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 2c51e9748a6a..49f490bae030 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -644,10 +644,9 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>> if (!pmd_present(pmdval)) {
>> if (likely(!(flags & FOLL_MIGRATION)))
>> return no_page_table(vma, flags);
>> - VM_BUG_ON(thp_migration_supported() &&
>> + VM_BUG_ON(!thp_migration_supported() ||
>
> I don't get this. thp_migration_supported() evaluates to a
> compile-time constant. What is the point in doing a runtime BUG based
> on a thing which is known at compile-time?
>
Yes, support thp migration is compile-time known. The exsiting VM_BUG_ON will
be nothing if thp_migration_supported() equal to false, so pmdval other than
migration entry will not be caught.
Now, with the patch, if thp_migration_supported() equal to false, we will have
VM_BUG_ON(1) so to catch above error case.
>> !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
end of thread, other threads:[~2021-12-09 1:41 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2021-12-08 13:39 [PATCH] mm/gup.c: stricter check on THP migration entry during follow_pmd_mask Li Xinhai
2021-12-08 23:35 ` Andrew Morton
2021-12-09 1:40 ` Li Xinhai
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