Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Sep 23, 2020 at 11:24:09AM -0400, Peter Xu wrote:
> On Tue, Sep 22, 2020 at 09:05:05AM -0300, Jason Gunthorpe wrote:
> > On Mon, Sep 21, 2020 at 05:20:31PM -0400, Peter Xu wrote:
> > > Pinned pages shouldn't be write-protected when fork() happens, because follow
> > > up copy-on-write on these pages could cause the pinned pages to be replaced by
> > > random newly allocated pages.
> > > 
> > > For huge PMDs, we split the huge pmd if pinning is detected.  So that future
> > > handling will be done by the PTE level (with our latest changes, each of the
> > > small pages will be copied).  We can achieve this by let copy_huge_pmd() return
> > > -EAGAIN for pinned pages, so that we'll fallthrough in copy_pmd_range() and
> > > finally land the next copy_pte_range() call.
> > > 
> > > Huge PUDs will be even more special - so far it does not support anonymous
> > > pages.  But it can actually be done the same as the huge PMDs even if the split
> > > huge PUDs means to erase the PUD entries.  It'll guarantee the follow up fault
> > > ins will remap the same pages in either parent/child later.
> > > 
> > > This might not be the most efficient way, but it should be easy and clean
> > > enough.  It should be fine, since we're tackling with a very rare case just to
> > > make sure userspaces that pinned some thps will still work even without
> > > MADV_DONTFORK and after they fork()ed.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > >  mm/huge_memory.c | 26 ++++++++++++++++++++++++++
> > >  1 file changed, 26 insertions(+)
> > > 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 7ff29cc3d55c..c40aac0ad87e 100644
> > > +++ b/mm/huge_memory.c
> > > @@ -1074,6 +1074,23 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > >  
> > >  	src_page = pmd_page(pmd);
> > >  	VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
> > > +
> > > +	/*
> > > +	 * If this page is a potentially pinned page, split and retry the fault
> > > +	 * with smaller page size.  Normally this should not happen because the
> > > +	 * userspace should use MADV_DONTFORK upon pinned regions.  This is a
> > > +	 * best effort that the pinned pages won't be replaced by another
> > > +	 * random page during the coming copy-on-write.
> > > +	 */
> > > +	if (unlikely(READ_ONCE(src_mm->has_pinned) &&
> > > +		     page_maybe_dma_pinned(src_page))) {
> > > +		pte_free(dst_mm, pgtable);
> > > +		spin_unlock(src_ptl);
> > > +		spin_unlock(dst_ptl);
> > > +		__split_huge_pmd(vma, src_pmd, addr, false, NULL);
> > > +		return -EAGAIN;
> > > +	}
> > 
> > Not sure why, but the PMD stuff here is not calling is_cow_mapping()
> > before doing the write protect. Seems like it might be an existing
> > bug?
> 
> IMHO it's not a bug, because splitting a huge pmd should always be safe.

Sur splitting is safe, but testing has_pinned without checking COW is
not, for what Jann explained.

The 'maybe' in page_maybe_dma_pinned() means it can return true when
the correct answer is false. It can never return false when the
correct answer is true.

It is the same when has_pinned is involved, the combined expression
must never return false when true is correct. Which means it can only
be applied for COW cases.

Jason