Re: [RFC 3/8] mm: Avoid using set_page_count() in set_page_recounted()

[Pasha Tatashin]

Hi John,

Thank you for looking at this series.

> >   static inline void set_page_refcounted(struct page *page)
> >   {
> > +     int refcnt;
> > +
> >       VM_BUG_ON_PAGE(PageTail(page), page);
> >       VM_BUG_ON_PAGE(page_ref_count(page), page);
> > -     set_page_count(page, 1);
> > +     refcnt = page_ref_inc_return(page);
> > +     VM_BUG_ON_PAGE(refcnt != 1, page);


> I am acutely uncomfortable with this change, because it changes the
> meaning and behavior of the function to something completely different,
> while leaving the function name unchanged. Furthermore, in relies upon
> debug assertions, rather than a return value (for example) to verify
> that all is well.


It must return the same thing, if it does not we have a bug in our
kernel which may lead to memory corruptions and security holes.

So today we have this:
   VM_BUG_ON_PAGE(page_ref_count(page), page); -> check ref_count is 0
   < What if something modified here? Hmm..>
   set_page_count(page, 1); -> Yet we reset it to 1.

With my proposed change:
   VM_BUG_ON_PAGE(page_ref_count(page), page); -> check ref_count is 0
   refcnt = page_ref_inc_return(page);  -> ref_count better be 1.
   VM_BUG_ON_PAGE(refcnt != 1, page); -> Verify that it is 1.

>
> I understand where this patchset is going, but this intermediate step is
> not a good move.
>
> Also, for the overall series, if you want to change from
> "set_page_count()" to "inc_and_verify_val_equals_one()", then the way to
> do that is *not* to depend solely on VM_BUG*() to verify. Instead,
> return something like -EBUSY if incrementing the value results in a
> surprise, and let the caller decide how to handle it.

Actually, -EBUSY would be OK if the problems were because we failed to
modify refcount for some reason, but if we modified refcount and got
an unexpected value (i.e underflow/overflow) we better report it right
away instead of waiting for memory corruption to happen.

Thanks,
Pasha