Re: [PATCH V5 0/9] Fixes for vhost metadata acceleration

On Thu, Aug 15, 2019 at 11:26:46AM +0800, Jason Wang wrote:

On 2019/8/13 下午7:57, Jason Gunthorpe wrote:

quoted

On Tue, Aug 13, 2019 at 04:31:07PM +0800, Jason Wang wrote:

quoted

What kind of issues do you see? Spinlock is to synchronize GUP with MMU
notifier in this series.

A GUP that can't sleep can't pagefault which makes it a really weird
pattern

My understanding is __get_user_pages_fast() assumes caller can fail or have
fallback. And we have graceful fallback to copy_{to|from}_user().

My point is that if you can fall back to copy_user then it is weird to
call the special non-sleeping GUP under a spinlock.

AFAIK the only reason this is done is because of the way the notifier
is being locked...

Jason