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SubjectRe: process 'stuck' at exit.
On Wed, 11 Dec 2013, Oleg Nesterov wrote:
> On 12/10, Linus Torvalds wrote:
> >
> > I think what happens is:
> > - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only)
> > - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page
> > - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only).
> >
> > so what triggers this is likely that Dave now does large-pages, and
> > one of them is a read-only mapping.
> >
> > So I would suggest replacing the second "1" in the
> > __get_user_pages_fast() call with a "!ro" instead. So how about this
> > second patch instead (the access_ok() move remains).
>
> I know almost nothing about THP, but why we may need write == true in
> this case?
>
> IOW,
>
> > - if (likely(__get_user_pages_fast(address, 1, 1, &page) == 1)) {
> > + if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
>
> can't
> if (likely(__get_user_pages_fast(address, 1, 0, &page) == 1)) {
>
> work?

If the futex call needs to write to that address, we need a writeable
mapping, right? And get_user_pages_fast() will verify that for us.

> I have to admit, I do not understand why we can't avoid this altogether.
>
> __get_page_tail() can find the stable ->first_page, why get_futex_key()
> can't ?

Because it can be split under us.

Thanks,

tglx


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