2022-09-06 03:08:14

by Chen Wandun

Subject: [PATCH] mm: avoid unnecessary page table walk for __get_user_pages

It is no need to walk page table and find pages if faultin_page success
and __get_user_pages does't care pages at all, so directly handle next
page.

Signed-off-by: Chen Wandun <[email protected]>
---
mm/gup.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c
index 983e24fd4b70..c8de33cc37af 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1198,7 +1198,10 @@ static long __get_user_pages(struct mm_struct *mm,
PTR_ERR(page) == -EMLINK, locked);
switch (ret) {
case 0:
- goto retry;
+ if (pages)
+ goto retry;
+ else
+ goto next_page;
case -EBUSY:
case -EAGAIN:
ret = 0;
--
2.25.1

2022-09-08 00:00:06

by Andrew Morton

Subject: Re: [PATCH] mm: avoid unnecessary page table walk for __get_user_pages

On Tue, 6 Sep 2022 10:44:01 +0800 Chen Wandun <[email protected]> wrote:

> It is no need to walk page table and find pages if faultin_page success
> and __get_user_pages does't care pages at all, so directly handle next
> page.
>
> ...
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1198,7 +1198,10 @@ static long __get_user_pages(struct mm_struct *mm,
> PTR_ERR(page) == -EMLINK, locked);
> switch (ret) {
> case 0:
> - goto retry;
> + if (pages)
> + goto retry;
> + else
> + goto next_page;
> case -EBUSY:
> case -EAGAIN:
> ret = 0;

Looks right to me. I redid the changelog somewhat:

: There is no need to walk page tables and find pages if faultin_page)_
: succeeds and __get_user_pages)_ doesn't care about populating the pages[]
: array. So directly go on to handle the next page.

2022-09-08 02:35:49

by John Hubbard

Subject: Re: [PATCH] mm: avoid unnecessary page table walk for __get_user_pages

On 9/7/22 16:34, Andrew Morton wrote:
> On Tue, 6 Sep 2022 10:44:01 +0800 Chen Wandun <[email protected]> wrote:
>
>> It is no need to walk page table and find pages if faultin_page success
>> and __get_user_pages does't care pages at all, so directly handle next
>> page.
>>
>> ...
>>
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -1198,7 +1198,10 @@ static long __get_user_pages(struct mm_struct *mm,
>> PTR_ERR(page) == -EMLINK, locked);
>> switch (ret) {
>> case 0:
>> - goto retry;
>> + if (pages)
>> + goto retry;
>> + else
>> + goto next_page;

Wow, good point. And it's been doing that for a *long* time!

>> case -EBUSY:
>> case -EAGAIN:
>> ret = 0;
>
> Looks right to me. I redid the changelog somewhat:
>
> : There is no need to walk page tables and find pages if faultin_page)_
> : succeeds and __get_user_pages)_ doesn't care about populating the pages[]
> : array. So directly go on to handle the next page.
>
>

With that re-worded commit description, please feel to add

Reviewed-by: John Hubbard <[email protected]>


thanks,
--
John Hubbard
NVIDIA