Re: CMA broken in next-20120926

[Thierry Reding]

On Fri, Sep 28, 2012 at 12:07:12PM +0100, Mel Gorman wrote:
> On Fri, Sep 28, 2012 at 12:51:13PM +0200, Thierry Reding wrote:
> > On Fri, Sep 28, 2012 at 12:38:15PM +0200, Thierry Reding wrote:
> > > On Fri, Sep 28, 2012 at 12:32:07PM +0200, Thierry Reding wrote:
> > > > On Fri, Sep 28, 2012 at 11:27:28AM +0100, Mel Gorman wrote:
> > > > > On Fri, Sep 28, 2012 at 11:48:25AM +0300, Peter Ujfalusi wrote:
> > > > > > Hi,
> > > > > > 
> > > > > > On 09/28/2012 11:37 AM, Mel Gorman wrote:
> > > > > > >> I hope this patch fixes the bug. If this patch fixes the problem
> > > > > > >> but has some problem about description or someone has better idea,
> > > > > > >> feel free to modify and resend to akpm, Please.
> > > > > > >>
> > > > > > > 
> > > > > > > A full revert is overkill. Can the following patch be tested as a
> > > > > > > potential replacement please?
> > > > > > > 
> > > > > > > ---8<---
> > > > > > > mm: compaction: Iron out isolate_freepages_block() and isolate_freepages_range() -fix1
> > > > > > > 
> > > > > > > CMA is reported to be broken in next-20120926. Minchan Kim pointed out
> > > > > > > that this was due to nr_scanned != total_isolated in the case of CMA
> > > > > > > because PageBuddy pages are one scan but many isolations in CMA. This
> > > > > > > patch should address the problem.
> > > > > > > 
> > > > > > > This patch is a fix for
> > > > > > > mm-compaction-acquire-the-zone-lock-as-late-as-possible-fix-2.patch
> > > > > > > 
> > > > > > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > > > > > 
> > > > > > linux-next + this patch alone also works for me.
> > > > > > 
> > > > > > Tested-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
> > > > > 
> > > > > Thanks Peter. I expect it also works for Thierry as I expect you were
> > > > > suffering the same problem but obviously confirmation of that would be nice.
> > > > 
> > > > I've been running a few tests and indeed this solves the obvious problem
> > > > that the coherent pool cannot be created at boot (which in turn caused
> > > > the ethernet adapter to fail on Tegra).
> > > > 
> > > > However I've been working on the Tegra DRM driver, which uses CMA to
> > > > allocate large chunks of framebuffer memory and these are now failing.
> > > > I'll need to check if Minchan's patch solves that problem as well.
> > > 
> > > Indeed, with Minchan's patch the DRM can allocate the framebuffer
> > > without a problem. Something else must be wrong then.
> > 
> > However, depending on the size of the allocation it also happens with
> > Minchan's patch. What I see is this:
> > 
> > [   60.736729] alloc_contig_range test_pages_isolated(1e900, 1f0e9) failed
> > [   60.743572] alloc_contig_range test_pages_isolated(1ea00, 1f1e9) failed
> > [   60.750424] alloc_contig_range test_pages_isolated(1ea00, 1f2e9) failed
> > [   60.757239] alloc_contig_range test_pages_isolated(1ec00, 1f3e9) failed
> > [   60.764066] alloc_contig_range test_pages_isolated(1ec00, 1f4e9) failed
> > [   60.770893] alloc_contig_range test_pages_isolated(1ec00, 1f5e9) failed
> > [   60.777698] alloc_contig_range test_pages_isolated(1ec00, 1f6e9) failed
> > [   60.784526] alloc_contig_range test_pages_isolated(1f000, 1f7e9) failed
> > [   60.791148] drm tegra: Failed to alloc buffer: 8294400
> > 
> > I'm pretty sure this did work before next-20120926.
> > 
> 
> Can you double check this please?
> 
> This is a separate bug but may be related to the same series. However, CMA should
> be ignoring the "skip" hints and because it's sync compaction it should
> not be exiting due to lock contention. Maybe Marek will spot it.

I've written a small test module that tries to allocate growing blocks
of contiguous memory and it seems like with your patch this always fails
at 8 MiB. Given the default size for CMA allocations is 16 MiB I thought
I should try increasing that but it doesn't make a difference. The
allocation still fails at 8 MiB.

Maybe that'll give another clue.

> Failing that, would you be in a position to bisect between v3.6-rc6 and
> current next to try pin-point exactly which patch introduced this
> problem please?

I can possibly do that but probably not before sometime later next week
unfortunately. But I'll see if I can find some time to double-check that
some older linux-next version works and that my memory isn't failing me.

Thierry

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