Messages in this thread | | | Date | Thu, 27 Jan 2011 14:30:21 +0000 | From | "Jan Beulich" <> | Subject | Re: One (possible) x86 get_user_pages bug |
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>>> On 27.01.11 at 14:56, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > On Thu, 2011-01-27 at 21:05 +0800, Xiaowei Yang wrote: >> >> However, from the comments embedded in gup.c, it seems deliberate to >> avoid the lock in the fast path. The question is: if so, how to avoid >> the above scenario? > > Something like the below comes to mind... but I must say I haven't fully > considered the problem yet..
That doesn't seem to account for the possible case of the page even managing to get allocated again to something else.
And I think you would need to drop out of gup_pte_range() in that case.
I would think this needs to be get_page_unless_zero() followed by re-checking of the page table entry (probably not even requiring a second gup_get_pte()); I'm not sure yet what the correct action would be for change in only the accessed/dirty bits.
Jan
> --- > arch/x86/mm/gup.c | 9 +++++---- > 1 files changed, 5 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c > index dbe34b9..6527933 100644 > --- a/arch/x86/mm/gup.c > +++ b/arch/x86/mm/gup.c > @@ -89,10 +89,11 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned > long addr, > } > VM_BUG_ON(!pfn_valid(pte_pfn(pte))); > page = pte_page(pte); > - get_page(page); > - SetPageReferenced(page); > - pages[*nr] = page; > - (*nr)++; > + if (get_page_unless_zero(page)) { > + SetPageReferenced(page); > + pages[*nr] = page; > + (*nr)++; > + } > > } while (ptep++, addr += PAGE_SIZE, addr != end); > pte_unmap(ptep - 1);
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