Re: [PATCH] [suggestion] mm/gup: avoid IS_ERR_OR_NULL

[John Hubbard]

On 5/28/23 09:22, Jason Gunthorpe wrote:
> On Sun, May 28, 2023 at 04:13:44PM +0100, Lorenzo Stoakes wrote:
> > On Sat, May 27, 2023 at 06:52:01AM -0300, Jason Gunthorpe wrote:
> > > On Sat, May 20, 2023 at 10:12:40AM +0100, Lorenzo Stoakes wrote:
> > > > > No, this returning 0 on failure is a foot gun. Failing to pin a single
> > > > > page is always an error, the only question is what sub reason caused
> > > > > the error to happen. There is no third case where it is not an error.
> > > >
> > > > The uprobe path thinks otherwise, but maybe the answer is that we just need
> > > > to -EFAULT on missing VMA and -EPERM on invalid flags.
> > >
> > > I think uprobe is just broken to think there is a third outcome. Let's
> > > just fix it instead of trying to pretend it makes sense.
> >
> > Sure, will take a look at that if I get a chance. We can at the very least
> > adjust get_user_page_vma_remote() with this fixed.

Great!

We've had previous discussions about getting rid of this pseudo-tristate
errno in gup, so I just wanted to mention that I'm also glad to see the
movement toward, "return some pages, or else a -errno". That's progress.


> > Do you feel that a partially successful pinning for other GUP callers
> > should equally be treated as an error (and pages unpinned -> return error
> > code)? In that instance we'd need to audit things somewhat.
>
> That seems more deeply ingrained at least, I'm not as keen to change
> it as to get rid of the 0 return result.

Yes. It's not just "audit things somewhat", it's more like, "change
quite a few call sites, some of which actually gather sets of partial
results in a retry loop". So some actual coding changes there.


thanks,
--
John Hubbard
NVIDIA