Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

[Jason Gunthorpe]

On Fri, Apr 28, 2023 at 11:56:55AM -0400, Peter Xu wrote:

> > PageAnon(page) can be called from GUP-fast after grabbing a reference. See
> > gup_must_unshare().
> 
> Hmm.. Is it a good idea at all to sacrifise all "!anon" fast-gups for this?
> People will silently got degrade even on legal pins on shmem/hugetlb, I
> think, which seems to be still a very major use case.

Remember gup fast was like this until quite recently - DAX wrecked it.

I fixed it when I changed DAX to not post-scan the VMA list..

I'm not sure longterm and really fast need to go together.

Jason