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.
Cheers,
Will
