Re: [PATCH 07/19] mm/fork: Accept huge pfnmap entries

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 09, 2024 at 06:32:44PM +0200, David Hildenbrand wrote:
> On 09.08.24 18:08, Peter Xu wrote:
> > Teach the fork code to properly copy pfnmaps for pmd/pud levels.  Pud is
> > much easier, the write bit needs to be persisted though for writable and
> > shared pud mappings like PFNMAP ones, otherwise a follow up write in either
> > parent or child process will trigger a write fault.
> > 
> > Do the same for pmd level.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   mm/huge_memory.c | 27 ++++++++++++++++++++++++---
> >   1 file changed, 24 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 6568586b21ab..015c9468eed5 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1375,6 +1375,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> >   	pgtable_t pgtable = NULL;
> >   	int ret = -ENOMEM;
> > +	pmd = pmdp_get_lockless(src_pmd);
> > +	if (unlikely(pmd_special(pmd))) {
> > +		dst_ptl = pmd_lock(dst_mm, dst_pmd);
> > +		src_ptl = pmd_lockptr(src_mm, src_pmd);
> > +		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> > +		/*
> > +		 * No need to recheck the pmd, it can't change with write
> > +		 * mmap lock held here.
> > +		 */
> > +		if (is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) {
> > +			pmdp_set_wrprotect(src_mm, addr, src_pmd);
> > +			pmd = pmd_wrprotect(pmd);
> > +		}
> > +		goto set_pmd;
> > +	}
> > +
> 
> I strongly assume we should be using using vm_normal_page_pmd() instead of
> pmd_page() further below. pmd_special() should be mostly limited to GUP-fast
> and vm_normal_page_pmd().

One thing to mention that it has this:

	if (!vma_is_anonymous(dst_vma))
		return 0;

So it's only about anonymous below that.  In that case I feel like the
pmd_page() is benign, and actually good.

Though what you're saying here made me notice my above check doesn't seem
to be necessary, I mean, "(is_cow_mapping(src_vma->vm_flags) &&
pmd_write(pmd))" can't be true when special bit is set, aka, pfnmaps.. and
if it's writable for CoW it means it's already an anon.

I think I can probably drop that line there, perhaps with a
VM_WARN_ON_ONCE() making sure it won't happen.

> 
> Again, we should be doing this similar to how we handle PTEs.
> 
> I'm a bit confused about the "unlikely(!pmd_trans_huge(pmd)" check, below:
> what else should we have here if it's not a migration entry but a present
> entry?

I had a feeling that it was just a safety belt since the 1st day of thp
when Andrea worked that out, so that it'll work with e.g. file truncation
races.

But with current code it looks like it's only anonymous indeed, so looks
not possible at least from that pov.

Thanks,

> 
> Likely this function needs a bit of rework.
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Peter Xu