linux-kernel - Re: [PATCH] mm, thp: fix locking inconsistency in collapse_huge

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Date:	Mon, 6 Jun 2016 15:05:19 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	akpm@...ux-foundation.org
Cc:	sergey.senozhatsky.work@...il.com, mhocko@...nel.org,
	kirill.shutemov@...ux.intel.com, sfr@...b.auug.org.au,
	linux-mm@...ck.org, linux-next@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com, aarcange@...hat.com
Subject: Re: [PATCH] mm, thp: fix locking inconsistency in collapse_huge_page

On 06/03/2016 02:28 PM, Ebru Akagunduz wrote:
> After creating revalidate vma function, locking inconsistency occured
> due to directing the code path to wrong label. This patch directs
> to correct label and fix the inconsistency.
>
> Related commit that caused inconsistency:
> http://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=da4360877094368f6dfe75bbe804b0f0a5d575b0
>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@...il.com>

I think this does fix the inconsistency, thanks.

But looking at collapse_huge_page() as of latest -next, I wonder if 
there's another problem:

pmd = mm_find_pmd(mm, address);
...
up_read(&mm->mmap_sem);
down_write(&mm->mmap_sem);
hugepage_vma_revalidate(mm, address);
...
pte = pte_offset_map(pmd, address);

What guarantees that 'pmd' is still valid?

Vlastimil