Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

[Christoph Hellwig]

I'm pretty sure DIRECT I/O reads that write into file backed mappings
are out there in the wild.

So while I wish we had never allowed this, the exercise seems futile and
instead we need to work on supporting this usecase, with the FOLL_PIN
infrastructure being a big step toward that.