Re: [RFC PATCH 4/8] mm: Separate fault info out of 'struct vm_fault'

[Will Deacon]

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