Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

From: Jason Gunthorpe
Date: Thu Sep 17 2020 - 18:09:04 EST

On Thu, Sep 17, 2020 at 05:40:59PM -0400, Peter Xu wrote:
> On Thu, Sep 17, 2020 at 01:35:56PM -0700, Linus Torvalds wrote:
> > For that to happen, we'd need to have the vma flag so that we wouldn't
> > have any worry about non-pinners, but as you suggested, I think even
> > just a mm-wide counter - or flag - to deal with the fast-bup case is
> > likely perfectly sufficient.
> Would mm_struct.pinned_vm suffice?

I think that could be a good long term goal

IIRC last time we dug into the locked_vm vs pinned_vm mess it didn't
get fixed. There is a mix of both kinds, as you saw, and some
resistance I don't clearly remember to changing it.

My advice for this -rc fix is to go with a single bit in the mm_struct
set on any call to pin_user_pages*

Then only users using pin_user_pages and forking are the only ones who
would ever do extra COW on fork. I think that is OK for -rc, this
workload should be rare due to the various historical issues. Anyhow,
a slow down regression is better than a it is broken regression.

This can be improved into a counter later. Due to the pinned_vm
accounting all call sites should have the mm_struct at unpin, but I
have a feeling it will take a alot of driver patches to sort it all