On Thu, Jan 14, 2021 at 11:41 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Thu, Jan 14, 2021 at 11:09:01AM -0800, Linus Torvalds wrote:
> > On Thu, Jan 14, 2021 at 11:00 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > >
> > > I tried that initially, but I found that I had to make all of the
> > > members const to get it to work, at which point the anonymous struct
> > > wasn't really adding anything. Did I just botch the syntax?
> >
> > I'm not sure what you tried. But this stupid test-case sure works for me:
> >
> > struct hello {
> > const struct {
> > unsigned long address;
> > };
> > unsigned int flags;
> > };
> >
> > extern int fn(struct hello *);
> >
> > int test(void)
> > {
> > struct hello a = {
> > .address = 1,
> > };
> > a.flags = 0;
> > return fn(&a);
> > }
> >
> > and because "address" is in that unnamed constant struct, you can only
> > set it within that initializer, and cannot do
> >
> > a.address = 0;
> >
> > without an error (the way you _can_ do "a.flags = 0").
> >
> > I don't see naming the struct making a difference - apart from forcing
> > that big rename patch, of course.
> >
> > But maybe we're talking about different issues?
>
> Urgh...
>
> We _are_ both on the same page, and your reply above had me thinking I've
> lost the plot, so I went back to the start. Check out v5.11-rc3 and apply
> this patch:
>
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ecdf8a8cd6ae..1eb950865450 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -514,11 +514,14 @@ static inline bool fault_flag_allow_retry_first(unsigned int flags)
> * pgoff should be used in favour of virtual_address, if possible.
> */
> struct vm_fault {
> - struct vm_area_struct *vma; /* Target VMA */
> + const struct {
> + struct vm_area_struct *vma; /* Target VMA */
> + gfp_t gfp_mask; /* gfp mask to be used for allocations */
> + pgoff_t pgoff; /* Logical page offset based on vma */
> + unsigned long address; /* Faulting virtual address */
> + };
> +
> unsigned int flags; /* FAULT_FLAG_xxx flags */
> - gfp_t gfp_mask; /* gfp mask to be used for allocations */
> - pgoff_t pgoff; /* Logical page offset based on vma */
> - unsigned long address; /* Faulting virtual address */
> pmd_t *pmd; /* Pointer to pmd entry matching
> * the 'address' */
> pud_t *pud; /* Pointer to pud entry matching
>
>
> Sure enough, an arm64 defconfig builds perfectly alright with that change,
> but it really shouldn't. I'm using clang 11.0.5, so I had another go with
> GCC 9.2.1 and bang:
>
> mm/filemap.c: In function ‘filemap_map_pages’:
> mm/filemap.c:2963:16: error: assignment of member ‘address’ in read-only object
> 2963 | vmf->address += (xas.xa_index - last_pgoff) << PAGE_SHIFT;
> | ^~
> make[1]: *** [scripts/Makefile.build:279: mm/filemap.o] Error 1
> make[1]: *** Waiting for unfinished jobs....
> make: *** [Makefile:1805: mm] Error 2
> make: *** Waiting for unfinished jobs....
>
> Nick -- any clue what's happening here? We would like that const anonymous
> struct to behave like a const struct member, as the alternative (naming the
> thing) results in a lot of refactoring churn.
Weird, looks like a bug to me in Clang, filed
https://bugs.llvm.org/show_bug.cgi?id=48755.
--
Thanks,
~Nick Desaulniers
