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
