linux-kernel - Re: [PATCH v2 2/2] hugetlbfs: Use i_mmap_rwsem to fix page fault/truncate race

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Message-ID: <20181221202136.crrwojz3k7muvyrh@kshutemo-mobl1>
Date:   Fri, 21 Dec 2018 23:21:37 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 2/2] hugetlbfs: Use i_mmap_rwsem to fix page
 fault/truncate race

On Fri, Dec 21, 2018 at 10:28:25AM -0800, Mike Kravetz wrote:
> On 12/21/18 2:28 AM, Kirill A. Shutemov wrote:
> > On Tue, Dec 18, 2018 at 02:35:57PM -0800, Mike Kravetz wrote:
> >> Instead of writing the required complicated code for this rare
> >> occurrence, just eliminate the race.  i_mmap_rwsem is now held in read
> >> mode for the duration of page fault processing.  Hold i_mmap_rwsem
> >> longer in truncation and hold punch code to cover the call to
> >> remove_inode_hugepages.
> > 
> > One of remove_inode_hugepages() callers is noticeably missing --
> > hugetlbfs_evict_inode(). Why?
> > 
> > It at least deserves a comment on why the lock rule doesn't apply to it.
> 
> In the case of hugetlbfs_evict_inode, the vfs layer guarantees there are
> no more users of the inode/file.

I'm not convinced that it is true. See documentation for ->evict_inode()
in Documentation/filesystems/porting:

	Caller does *not* evict the pagecache or inode-associated
	metadata buffers; the method has to use truncate_inode_pages_final() to get rid
	of those.

Is hugetlbfs special here?

-- 
 Kirill A. Shutemov