Re: [PATCH v9 0/3] mm/gup: disallow GUP writing to file-backed mappings by default

[David Hildenbrand]

On 15.05.23 13:31, Lorenzo Stoakes wrote:
> On Sun, May 14, 2023 at 10:14:46PM -0700, Christoph Hellwig wrote:
> > On Sun, May 14, 2023 at 08:20:04PM +0100, Lorenzo Stoakes wrote:
> > > As discussed at LSF/MM, on the flight over I wrote a little repro [0] which
> > > reliably triggers the ext4 warning by recreating the scenario described
> > > above, using a small userland program and kernel module.
> > >
> > > This code is not perfect (plane code :) but does seem to do the job
> > > adequately, also obviously this should only be run in a VM environment
> > > where data loss is acceptable (in my case a small qemu instance).
> >
> > It would be really awesome if you could wire it up with and submit it
> > to xfstests.
>
> Sure am happy to take a look at that! Also happy if David finds it useful in any
> way for this unit tests.

I played with a simple selftest that would reuse the existing gup_test 
infrastructure (adding PIN_LONGTERM_TEST_WRITE), and try reproducing an 
actual data corruption.

So far, I was not able to reproduce any corruption easily without your 
patches, because d824ec2a1546 ("mm: do not reclaim private data from 
pinned page") seems to mitigate most of it.

So ... before my patches (adding PIN_LONGTERM_TEST_WRITE) I cannot test 
it from a selftest, with d824ec2a1546 ("mm: do not reclaim private data 
from pinned page") I cannot reproduce and with your patches long-term 
pinning just fails.

Long story short: I'll most probably not add such a test but instead 
keep testing that long-term pinning works/fails now as expected, based 
on the FS type.

> The kernel module interface is a bit sketchy (it takes a user address which it
> blindly pins for you) so it's not something that should be run in any unsafe
> environment but as long as we are ok with that :)

I can submit the PIN_LONGTERM_TEST_WRITE extension, that would allow to 
test with a stock kernel that has the module compiled in. It won't allow 
!longterm, though (it would be kind-of hacky to have !longterm 
controlled by user space, even if it's a GUP test module).

Finding an actual reproducer using existing pinning functionality would 
be preferred. For example, using O_DIRECT (should be possible even 
before it starts using FOLL_PIN instead of FOLL_GET). That would be 
highly racy then, but most probably not impossible.

Such (racy) tests are not a good fit for selftests.

Maybe I'll have a try later to reproduce with O_DIRECT.

--
Thanks,

David / dhildenb