Re: [RFC PATCH] mm: Fix a huge pud insertion race during faulting

[Dan Williams]

On Tue, Oct 15, 2019 at 3:06 AM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 08, 2019 at 11:37:11AM +0200, Thomas HellstrÃm (VMware) wrote:
> > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> >
> > A huge pud page can theoretically be faulted in racing with pmd_alloc()
> > in __handle_mm_fault(). That will lead to pmd_alloc() returning an
> > invalid pmd pointer. Fix this by adding a pud_trans_unstable() function
> > similar to pmd_trans_unstable() and check whether the pud is really stable
> > before using the pmd pointer.
> >
> > Race:
> > Thread 1:             Thread 2:                 Comment
> > create_huge_pud()                               Fallback - not taken.
> >                     create_huge_pud()         Taken.
> > pmd_alloc()                                     Returns an invalid pointer.
> >
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
> > Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > ---
> > RFC: We include pud_devmap() as an unstable PUD flag. Is this correct?
> >      Do the same for pmds?
>
> I *think* it is correct and we should do the same for PMD, but I may be
> wrong.
>
> Dan, Matthew, could you comment on this?

The _devmap() check in these paths near _trans_unstable() has always
been about avoiding assumptions that the corresponding page might be
page cache or anonymous which for dax it's neither and does not behave
like a typical page.