Re: [PATCH hmm 2/8] mm/hmm: don't free the cached pgmap while scanning

[Jason Gunthorpe <jgg@xxxxxxxx>]

On Mon, Mar 16, 2020 at 10:02:50AM +0100, Christoph Hellwig wrote:
> On Wed, Mar 11, 2020 at 03:35:00PM -0300, Jason Gunthorpe wrote:
> > @@ -694,6 +672,15 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
> >  			return -EBUSY;
> >  		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
> >  				      &hmm_walk_ops, &hmm_vma_walk);
> > +		/*
> > +		 * A pgmap is kept cached in the hmm_vma_walk to avoid expensive
> > +		 * searching in the probably common case that the pgmap is the
> > +		 * same for the entire requested range.
> > +		 */
> > +		if (hmm_vma_walk.pgmap) {
> > +			put_dev_pagemap(hmm_vma_walk.pgmap);
> > +			hmm_vma_walk.pgmap = NULL;
> > +		}
> >  	} while (ret == -EBUSY);
> 
> In which case it should only be put on return, and not for every loop.

I chose this to be simple without having to goto unwind it.

So, instead like this:

@@ -683,21 +661,33 @@ long hmm_range_fault(struct hmm_range *range, unsigned int flags)
 		.flags = flags,
 	};
 	struct mm_struct *mm = range->notifier->mm;
-	int ret;
+	long ret;
 
 	lockdep_assert_held(&mm->mmap_sem);
 
 	do {
 		/* If range is no longer valid force retry. */
 		if (mmu_interval_check_retry(range->notifier,
-					     range->notifier_seq))
-			return -EBUSY;
+					     range->notifier_seq)) {
+			ret = -EBUSY;
+			goto out;
+		}
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
 	} while (ret == -EBUSY);
 
 	if (ret)
-		return ret;
-	return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
+		goto out;
+	ret = (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
+
+out:
+	/*
+	 * A pgmap is kept cached in the hmm_vma_walk to avoid expensive
+	 * searching in the probably common case that the pgmap is the
+	 * same for the entire requested range.
+	 */
+	if (hmm_vma_walk.pgmap)
+		put_dev_pagemap(hmm_vma_walk.pgmap);
+	return ret;
 }
 EXPORT_SYMBOL(hmm_range_fault);

?

> I still think the right fix is to just delete all the unused and broken
> pgmap handling code.  If we ever need to add it back it can be added
> in a proper understood and tested way.

What I want to add is something like

 if (pgmap != walk->required_pgmap)
     cpu_flags = 0
 hmm_range_need_fault(..., cpu_flags, ...)

Which will fix a bug in nouveau where it blindly assumes any device
pages are its own, IIRC.

I think Ralph observed it needs to be here, because if the pgmap
doesn't match then it should trigger migration, in a single call,
rather than iterating.

I'm mostly expecting to replace all the other pgmap code, but keep the
pgmap caching. The existing pgmap stuff seems approx right, but
useless..

Jason
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