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SubjectRe: [PATCH v3 3/4] mm: don't expose non-hugetlb page to fast gup prematurely
On Thu 26-09-19 20:26:46, John Hubbard wrote:
> On 9/26/19 3:20 AM, Kirill A. Shutemov wrote:
> > BTW, have you looked at other levels of page table hierarchy. Do we have
> > the same issue for PMD/PUD/... pages?
> >
>
> Along the lines of "what other memory barriers might be missing for
> get_user_pages_fast(), I'm also concerned that the synchronization between
> get_user_pages_fast() and freeing the page tables might be technically broken,
> due to missing memory barriers on the get_user_pages_fast() side. Details:
>
> gup_fast() disables interrupts, but I think it also needs some sort of
> memory barrier(s), in order to prevent reads of the page table (gup_pgd_range,
> etc) from speculatively happening before the interrupts are disabled.

Could you be more specific about the race scenario please? I thought
that the unmap path will be serialized by the pte lock.
--
Michal Hocko
SUSE Labs

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