Messages in this thread | | | From | Pasha Tatashin <> | Date | Tue, 26 Oct 2021 14:21:53 -0400 | Subject | Re: [RFC 3/8] mm: Avoid using set_page_count() in set_page_recounted() |
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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
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