Hello,
Is there any reason to take mmap_sem explicitly here?
Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?
============================
down_read(&mm->mmap_sem);
get_user_pages(current, current->mm)
up_read(&mm->mmap_sem);
and
get_user_pages_fast()
make an equivalent result, And latter would be better when mamp_sem
highly contended case, because it can avoid to take mmap_sem if
the target page doesn't need a page fault.
Signed-off-by: KOSAKI Motohiro <[email protected]>
---
kernel/futex.c | 8 +++-----
1 files changed, 3 insertions(+), 5 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc2..9f7b1ae 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -351,13 +351,11 @@ static inline void put_futex_key(union futex_key *key)
*/
static int fault_in_user_writeable(u32 __user *uaddr)
{
- struct mm_struct *mm = current->mm;
int ret;
+ struct page *page;
- down_read(&mm->mmap_sem);
- ret = get_user_pages(current, mm, (unsigned long)uaddr,
- 1, 1, 0, NULL, NULL);
- up_read(&mm->mmap_sem);
+ ret = get_user_pages_fast((unsigned long)uaddr, 1, 1, &page);
+ put_page(page);
return ret < 0 ? ret : 0;
}
--
1.7.3.1
On 06/22/2011 11:21 PM, KOSAKI Motohiro wrote:
> Hello,
>
> Is there any reason to take mmap_sem explicitly here?
> Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?
>
> ============================
> down_read(&mm->mmap_sem);
> get_user_pages(current, current->mm)
> up_read(&mm->mmap_sem);
>
> and
>
> get_user_pages_fast()
>
> make an equivalent result, And latter would be better when mamp_sem
> highly contended case, because it can avoid to take mmap_sem if
> the target page doesn't need a page fault.
I can't speak authoritatively here, but it seems to me that
get_user_pages_fast falls back to get_user_pages with mmap_sem anyway,
so this seems like a reasonable optimization for the best case, with a
minor overhead for the slow case.
Peter, am I missing something? Is there a reason you left this as is
during your fast gup futex update?
>
> Signed-off-by: KOSAKI Motohiro <[email protected]>
> ---
> kernel/futex.c | 8 +++-----
> 1 files changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index fe28dc2..9f7b1ae 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -351,13 +351,11 @@ static inline void put_futex_key(union futex_key *key)
> */
> static int fault_in_user_writeable(u32 __user *uaddr)
> {
> - struct mm_struct *mm = current->mm;
> int ret;
> + struct page *page;
>
> - down_read(&mm->mmap_sem);
> - ret = get_user_pages(current, mm, (unsigned long)uaddr,
> - 1, 1, 0, NULL, NULL);
> - up_read(&mm->mmap_sem);
> + ret = get_user_pages_fast((unsigned long)uaddr, 1, 1, &page);
> + put_page(page);
>
> return ret < 0 ? ret : 0;
> }
--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
On Thu, 2011-06-23 at 08:11 -0700, Darren Hart wrote:
>
> On 06/22/2011 11:21 PM, KOSAKI Motohiro wrote:
> > Hello,
> >
> > Is there any reason to take mmap_sem explicitly here?
> > Should we change gup_fast() to allow NULL argument (ie for avoid get_page)?
> >
> > ============================
> > down_read(&mm->mmap_sem);
> > get_user_pages(current, current->mm)
> > up_read(&mm->mmap_sem);
> >
> > and
> >
> > get_user_pages_fast()
> >
> > make an equivalent result, And latter would be better when mamp_sem
> > highly contended case, because it can avoid to take mmap_sem if
> > the target page doesn't need a page fault.
>
>
> I can't speak authoritatively here, but it seems to me that
> get_user_pages_fast falls back to get_user_pages with mmap_sem anyway,
> so this seems like a reasonable optimization for the best case, with a
> minor overhead for the slow case.
>
> Peter, am I missing something?
Right, its much cheaper on the fast-path where the page is found present
and writable, if we have to take the fault, the extra cost in the slow
path is neglectable.
> Is there a reason you left this as is
> during your fast gup futex update?
I probably completely missed this gup() user.
The proposed change looks good to me.