Subject: Re: linux-next: manual merge of the mm tree with Linus' tree

From: Stephen Rothwell <[email-masked]>
Date: Fri, 28 Jul 2023 10:00:47 +1000

Hi all,

On Fri, 28 Jul 2023 09:29:15 +1000 Stephen Rothwell <[email-masked]> wrote:
>
> On Fri, 28 Jul 2023 09:18:49 +1000 Stephen Rothwell <[email-masked]> wrote:
> >
> > Today's linux-next merge of the mm tree got a conflict in:
> > 
> >   mm/memory.c
> > 
> > between commit:
> > 
> >   657b5146955e ("mm: lock_vma_under_rcu() must check vma->anon_vma under vma lock")
> > 
> > from Linus' tree and commits:
> > 
> >   69f6bbd1317f ("mm: handle userfaults under VMA lock")
> >   a3bdf38e85aa ("mm: allow per-VMA locks on file-backed VMAs")
> > 
> > from the mm tree.
> > 
> > I fixed it up (I think, please check - see below) and can carry the fix
> > as necessary. This is now fixed as far as linux-next is concerned, but
> > any non trivial conflicts should be mentioned to your upstream
> > maintainer when your tree is submitted for merging.  You may also want
> > to consider cooperating with the maintainer of the conflicting tree to
> > minimise any particularly complex conflicts.
> > 
> > -- 
> > Cheers,
> > Stephen Rothwell
> > 
> > diff --cc mm/memory.c
> > index ca632b58f792,271982fab2b8..000000000000
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@@ -5392,32 -5597,18 +5597,21 @@@ retry
> >   	if (!vma)
> >   		goto inval;
> >   
> > - 	/* Only anonymous and tcp vmas are supported for now */
> > - 	if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
> >  -	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
> >  -	if (vma_is_anonymous(vma) && !vma->anon_vma)
> > --		goto inval;
> > --
> >   	if (!vma_start_read(vma))
> >   		goto inval;
> >   
> >  +	/*
> >  +	 * find_mergeable_anon_vma uses adjacent vmas which are not locked.
> >  +	 * This check must happen after vma_start_read(); otherwise, a
> >  +	 * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
> >  +	 * from its anon_vma.
> >  +	 */
> > - 	if (unlikely(!vma->anon_vma && !vma_is_tcp(vma)))
> > - 		goto inval_end_read;
> > - 
> > - 	/*
> > - 	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
> > - 	 * it for now and fall back to page fault handling under mmap_lock.
> > - 	 */
> > - 	if (userfaultfd_armed(vma))
> > ++	if (unlikely(vma_is_anonymous(vma) && !vma_is_tcp(vma)))
> >  +		goto inval_end_read;
> >  +
> >   	/* Check since vm_start/vm_end might change before we lock the VMA */
> >  -	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
> >  -		vma_end_read(vma);
> >  -		goto inval;
> >  -	}
> >  +	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
> >  +		goto inval_end_read;
> >   
> >   	/* Check if the VMA got isolated after we found it */
> >   	if (vma->detached) {  
> 
> Sorry, doesn't even build ... let me try that again.

I have gone with below.  Again, please check.

-- 
Cheers,
Stephen Rothwell

diff --cc mm/memory.c
index ca632b58f792,271982fab2b8..000000000000
--- a/mm/memory.c
+++ b/mm/memory.c
@@@ -5392,32 -5597,18 +5597,21 @@@ retry
  	if (!vma)
  		goto inval;
  
- 	/* Only anonymous and tcp vmas are supported for now */
- 	if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
 -	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
 -	if (vma_is_anonymous(vma) && !vma->anon_vma)
--		goto inval;
--
  	if (!vma_start_read(vma))
  		goto inval;
  
 +	/*
 +	 * find_mergeable_anon_vma uses adjacent vmas which are not locked.
 +	 * This check must happen after vma_start_read(); otherwise, a
 +	 * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
 +	 * from its anon_vma.
 +	 */
- 	if (unlikely(!vma->anon_vma && !vma_is_tcp(vma)))
- 		goto inval_end_read;
- 
- 	/*
- 	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
- 	 * it for now and fall back to page fault handling under mmap_lock.
- 	 */
- 	if (userfaultfd_armed(vma))
++	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
 +		goto inval_end_read;
 +
  	/* Check since vm_start/vm_end might change before we lock the VMA */
 -	if (unlikely(address < vma->vm_start || address >= vma->vm_end)) {
 -		vma_end_read(vma);
 -		goto inval;
 -	}
 +	if (unlikely(address < vma->vm_start || address >= vma->vm_end))
 +		goto inval_end_read;
  
  	/* Check if the VMA got isolated after we found it */
  	if (vma->detached) {