Re: compound_head() vs uninitialized struct page poisoning

[Pavel Tatashin <pasha.tatashin@soleen.com>]

On Wed, May 1, 2019 at 4:24 PM Matthew Wilcox <willy@infradead.org> wrote:
>
>
> Hi Pavel,
>
> This strikes me as wrong:
>
> #define PF_HEAD(page, enforce)  PF_POISONED_CHECK(compound_head(page))
>
> If we hit a page which is poisoned, PAGE_POISON_PATTERN is ~0, so PageTail
> is set, and compound_head will return() 0xfff..ffe.  PagePoisoned()
> will then try to derefence that pointer and we'll get an oops that isn't
> obviously PagePoisoned.
>
> I think this should have been:
>
> #define PF_HEAD(page, enforce)  compound_head(PF_POISONED_CHECK(page))

Yes, I agree,  this makes sense.

>
> One could make the argument for double-checking:
>
> #define PF_HEAD(page, enforce)  PF_POISONED_CHECK(compound_head(PF_POISONED_CHECK(page)))
>
> but I think this is overkill; if a tail page is initialised, then there's
> no way that its head page should have been uninitialised.

Also agree, no need to check head if subpage is initialized.

>
> Would a patch something along these lines make sense?  Compile-tested only.

Yes, I like the re-ordering PF_POISONED_CHECK()s to  be before the
other accesses to PPs.

Thank you,
Pasha