On Fri, Oct 28, 2022 at 08:27:57AM -0700, Mike Kravetz wrote:

On 10/27/22 15:34, Peter Xu wrote:

quoted

On Wed, Oct 26, 2022 at 05:34:04PM -0700, Mike Kravetz wrote:

quoted

On 10/26/22 17:59, Peter Xu wrote:

If we want to use the vma read lock to protect here as the slow gup path,
then please check again with below [1] - I think we'll also need to protect
it with fast-gup (probably with trylock only, because fast-gup cannot
sleep) or it'll encounter the same race, iiuc.

Actually, instead of using vma lock, I really think this is another problem
and needs standalone fixing.  The problem is we allows huge_pte_offset() to
walk the process pgtable without any protection, while pmd unsharing can
drop a page anytime.  huge_pte_offset() is always facing use-after-free
when walking the PUD page.

We may want RCU lock to protect the pgtable pages from getting away when
huge_pte_offset() is walking it, it'll be safe then because pgtable pages
are released in RCU fashion only (e.g. in above example, process [2] will
munmap() and release the last ref to the "used to be shared" pmd and the
PUD that maps the shared pmds will be released only after a RCU grace
period), and afaict that's also what's protecting fast-gup from accessing
freed pgtable pages.

If with all huge_pte_offset() callers becoming RCU-safe, then IIUC we can
drop the vma lock in all GUP code, aka, in hugetlb_follow_page_mask() here,
because both slow and fast gup should be safe too in the same manner.

Thanks,

quoted

quoted

IIUC it's also the same as fast-gup - afaiu we don't take the read vma lock
in fast-gup too but I also think it's safe.  But I hope I didn't miss
something.

[1]

Thanks Peter!  I think the best thing would be to eliminate the vma_lock
calls in this patch.  The code it is replacing/simplifying does not do any
locking, so no real regression.

Agreed.

I think a scheme like you describe above is going to require some more
thought/work.  It might be better as a follow on patch.

So above is only a thought, but if you think it's so far not very wrong and
worth trying, I can see what I can get from it by some upcoming patches.

It shouldn't need a lot of change, but basically looking after all
huge_pte_offset() to make sure they're safe regarding walking the PUD.  I'm
attaching an initial patch to just start to comment on the usage of
huge_pte_offset() first because that'll be the gust of the upcoming
patchset (if there'll be), further comments welcomed too.  Thanks.

-- 
Peter Xu