Re: [PATCH v2 13/13] mm/gup: move private gup FOLL_ flags to internal.h

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jan 26, 2023 at 01:48:46PM +0100, David Hildenbrand wrote:
> On 24.01.23 21:34, Jason Gunthorpe wrote:
> > Move the flags that should not/are not used outside gup.c and related into
> > mm/internal.h to discourage driver abuse.
> > 
> > To make this more maintainable going forward compact the two FOLL ranges
> > with new bit numbers from 0 to 11 and 16 to 21, using shifts so it is
> > explict.
> > 
> > Switch to an enum so the whole thing is easier to read.
> 
> Using a __bitwise type would be even better, but that requires quite some
> adjustments ...
> 
> The primary leftover for FOLL_GET seems to be follow_page(). IIRC, there is
> only one caller that doesn't pass FOLL_GET (s390). We could either add a new
> function to "probe" that anything is mapped (IIRC that's the use case), or
> simply ref+unref.

Is that code even safe as written? I don't really understand how it
can safely call lock_page() on something it doesn't have a reference
too ?

So adding the FOLL_GET and put_page seems like a good idea to me? At a
minimum this should get a comment to explain why it is OK.

S390 people?

int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
{
[..]
        rc = -ENXIO;
        page = follow_page(vma, uaddr, FOLL_WRITE);
        if (IS_ERR_OR_NULL(page))
                goto out;

        lock_page(page);
        ptep = get_locked_pte(gmap->mm, uaddr, &ptelock);

Jason