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

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Date:	Thu, 9 Jun 2016 12:51:08 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	akpm@...ux-foundation.org, 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 (06/06/16 15:05), Vlastimil Babka wrote:
[..]
> 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?

the same question applied to __collapse_huge_page_swapin(), I think.

__collapse_huge_page_swapin(pmd)
	pte = pte_offset_map(pmd, address);
	do_swap_page(mm, vma, _address, pte, pmd...)
		up_read(&mm->mmap_sem);
	down_read(&mm->mmap_sem);
	pte = pte_offset_map(pmd, _address);

	-ss