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

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 24 Apr 2023 10:39:25 -0300

On Mon, Apr 24, 2023 at 01:38:49PM +0100, Lorenzo Stoakes wrote:

> I was being fairly conservative in that list, though we certainly need to
> set the flag for /proc/$pid/mem and ptrace to avoid breaking this
> functionality (I observed breakpoints breaking without it which obviously
> is a no go :). I'm not sure if there's a more general way we could check
> for this though?

More broadly we should make sure these usages of GUP safe somehow so
that it can reliably write to those types of pages without breaking
the current FS contract..

I forget exactly, but IIRC, don't you have to hold some kind of page
spinlock while writing to the page memory?

So, users that do this, or can be fixed to do this, can get file
backed pages. It suggests that a flag name is more like
FOLL_CALLER_USES_FILE_WRITE_LOCKING

> I wouldn't be totally opposed to dropping it for RDMA too, because I
> suspect accessing file-backed mappings for that is pretty iffy.
> 
> Do you have a sense of which in the list you feel could be pared back?

Anything using FOLL_LONGTERM should not set the flag, GUP should even
block the combination.

And we need to have in mind that the flag indicates the code is
buggy, so if you set it then we should understand how is that caller
expected to be fixed.

Jason