linux-kernel - Re: [PATCH] aoe: adjust ref of head for compound page tails

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Date:	Thu, 8 Aug 2013 17:02:27 -0400
From:	Ed Cashin <ecashin@...aid.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] aoe: adjust ref of head for compound page tails

On Aug 8, 2013, at 3:56 PM, Andrew Morton wrote:

> On Wed, 7 Aug 2013 20:50:09 -0400 Ed Cashin <ecashin@...aid.com> wrote:
...
>> When the workaround was created, it was with the assumption that the zero-count pages are not always tail pages, and that seemed to be the case in 2007, but as I said, I don't have a mechanism for detecting that now, so I cannot say whether it really happens with today's kernel.
> 
> It sounds we should pull out all that code and retest.  It shouldn't be
> needed - if this results in some failure then I suspect core MM will
> need changes.

OK.  I'll look into that.  It sure would be nice to get rid of it.

> Why don't you have a "mechanism for detecting that"?  It's a matter of
> pointing AOE at some hugetlb pages?

No, I was testing with hugetlb pages already.  But there are many use case combinations, and I meant that aoe has no mechanism that has been in the aoe code all these years to catch specific (possibly rare) use cases that result in zero-count pages.  It would be something like this:

+static void
+check_page_counts(struct bio *bio)
+{
+       struct bio_vec *bv;
+       int n;
+       int i;
+
+       bio_for_each_segment(bv, bio, i) {
+               n = page_count(bv->bv_page);
+               WARN_ONCE(n <= 0,
+                       "aoe: page %d in bio has non-positive count %d\n",
+                       i, n);
+       }
+}

I'll add that when I try testing without the page count manipulation.

-- 
  Ed Cashin
  ecashin@...aid.com


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