Re: [PATCH 3/4] coredump: ELF-FDPIC: enable to omit anonymous shared memory

[David Howells]

Hugh Dickins <hugh@xxxxxxxxxxx> wrote:

> > VMAs are a shared resource under NOMMU conditions.
> 
> That's a disturbing remark.

Why?  No-one complained when I first put up my rewrite patches three years ago.

> Under precisely what NOMMU conditions?

CONFIG_MMU=n.

> I had thought Robin's suggestion very sensible; and throughout mm/
> it has seemed pretty random whether we pass an "mm" argument down
> in addition to "vma", or just take vma->vm_mm at whatever level needs.
> 
> You seem to be suggesting vma->vm_mm is dangerous when CONFIG_NOMMU,

vm_mm is never set to anything other than NULL if CONFIG_MMU=n and it doesn't
seem to be a problem.  I don't think anything in the mm/ directory is left that
looks at vm_mm once MMU support is disabled (in fact I've just checked, and I
can compile with vm_mm #ifdef'd out)

> but we MMU people are scarily unaware of that.

If you're worryied that you can't compile anything for NOMMU, an FRV compiler
is available, and a suitable NOMMU default config can be provided.
Alternatively, you can pick ARM, M68K, ...

> Perhaps you need to put #ifndef CONFIG_NOMMU around vm_mm in struct
> vm_area_struct?

I can if it makes you happier.  It's not strictly necessary, but it does make
the struct smaller which is good.

David
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