Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

From: Yang Shi
Date: Tue Mar 27 2018 - 14:38:32 EST




On 3/27/18 2:29 AM, Michal Hocko wrote:
On Tue 27-03-18 02:20:39, Yang Shi wrote:
[...]
The patch looks reasonable to me. Maybe it would be better to be more
explicit about the purpose of the patch. As others noticed, this alone
wouldn't solve the mmap_sem contention issues. I _think_ that if you
were more explicit about the mmap_sem abuse it would trigger less
questions.

Yes, sure.


I have just one more question. Now that you are touching this area,
would you be willing to remove the following ugliness?

diff --git a/kernel/sys.c b/kernel/sys.c
index f2289de..17bddd2 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1959,7 +1959,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
return error;
}
- down_write(&mm->mmap_sem);
+ down_read(&mm->mmap_sem);
Why do we need to hold mmap_sem here and call find_vma, when only
PR_SET_MM_ENV_END: is consuming it? I guess we can replace it wit the
new lock and take the mmap_sem only for PR_SET_MM_ENV_END.

Actually, I didn't think of why. It looks prctl_set_mm() checks if vma does exist when it tries to set stack_start, argv_* and env_*, btw not only env_end.

Cyrill may be able to give us some hint since C/R is the main user of this API.

Yang


Thanks!