linux-kernel - RE: Hugepage regression

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Date:	Tue, 10 Oct 2006 20:18:24 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
cc:	"'David Gibson'" <david@...son.dropbear.id.au>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: RE: Hugepage regression

On Tue, 10 Oct 2006, Chen, Kenneth W wrote:
> 
> With the pending shared page table for hugetlb currently sitting in -mm,
> we serialize the all hugetlb unmap with a per file i_mmap_lock.  This
> race could well be solved by that pending patch?
> 
> http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.19-rc1/2.6.19-rc1-mm1/broken-out/shared-page-table-for-hugetlb-page-v4.patch

Hey, nice try, Ken!  But I don't think we can let you sneak shared
pagetables into 2.6.19 that way ;)

Sorry for not noticing this bug in your original TLB flush fix,
which had looked good to me.

Yes, I'd expect your i_mmap_lock to solve the problem: and since
you're headed in that direction anyway, it makes most sense to use
that solution rather than get into defining arrays, or sacrificing
the lazy flush, or risking page_count races.

So please extract the __unmap_hugepage_range mods from your shared
pagetable patch, and use that to fix the bug.  But again, I protest
the "if (vma->vm_file)" in your unmap_hugepage_range - how would a
hugepage area ever have NULL vma->vm_file?

Hugh
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