Re: [PATCH] arch/cris: use get_user_pages_fast()

[Davidlohr Bueso]

On Sun, 21 Jan 2018, Al Viro wrote:

> On Sun, Jan 21, 2018 at 02:59:29PM -0800, Davidlohr Bueso wrote:
> > Since we currently hold mmap_sem across both gup calls (and
> > nothing more), we can substitute it with two _fast()
> > alternatives and possibly avoid grabbing the lock.
> >
> > This was found while adding mmap_sem wrappers, and was also
> > previously reported by Al: https://lkml.org/lkml/2017/11/17/777
>
> See commit 9a949e8ff92246c0753b2805c2a001cb991fffe5
> Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Date:   Sat Nov 18 14:37:46 2017 -0500
>
>    cris: switch to get_user_pages_fast()
>
>    no point holding ->mmap_sem over both calls.
>
>    Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Serves me for developing on Linus' tree, cool.

Now, regarding the other two which are technically bugs (not
holding mmap_sem under gup), you mention I don't see those in
next, so I'm assuming you didn't get to them.

- mthca_map_user_db(): I came to the same conclusion and tempted
 to send out the same idea of unlocking the mutex for gup_fast()
 so we can take mmap_sem if needed.

- ia64's store_virtual_to_phys() is a sysfs attribute and
 holds no (other) locks. How about we just convert it as well?

diff --git a/arch/ia64/kernel/err_inject.c b/arch/ia64/kernel/err_inject.c
index 85bba43e7d5d..658a8e06a69b 100644
--- a/arch/ia64/kernel/err_inject.c
+++ b/arch/ia64/kernel/err_inject.c
@@ -142,7 +142,7 @@ store_virtual_to_phys(struct device *dev, struct device_attribute *attr,
	u64 virt_addr=simple_strtoull(buf, NULL, 16);
	int ret;

-	ret = get_user_pages(virt_addr, 1, FOLL_WRITE, NULL, NULL);
+	ret = get_user_pages_fast(virt_addr, 1, FOLL_WRITE, NULL);
	if (ret<=0) {
#ifdef ERR_INJ_DEBUG
		printk("Virtual address %lx is not existing.\n",virt_addr);


Thanks,
Davidlohr