Re: [PATCH v3] hugetlb: simplify hugetlb handling in follow_page_mask

[Peter Xu]

 On Fri, Oct 28, 2022 at 08:27:57AM -0700, Mike Kravetz wrote:
> On 10/27/22 15:34, Peter Xu wrote:
> > On Wed, Oct 26, 2022 at 05:34:04PM -0700, Mike Kravetz wrote:
> > > 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,
> > 
> > > > 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