Date: Mon, 11 Apr 2016 13:35:08 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andres Lagar-Cavilla <andreslc@google.com>,
        Yang Shi <yang.shi@linaro.org>, Ning Qu <quning@gmail.com>,
        Matthew Wilcox <willy@linux.intel.com>,
        David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 09/10] huge pagecache: mmap_sem is unlocked when
 truncation splits pmd
Message-ID: <20160411103508.GC22996@node.shutemov.name>
References: <alpine.LSU.2.11.1604051329480.5965@eggly.anvils>
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On Tue, Apr 05, 2016 at 01:55:23PM -0700, Hugh Dickins wrote:
> zap_pmd_range()'s CONFIG_DEBUG_VM !rwsem_is_locked(&mmap_sem) BUG()
> will be invalid with huge pagecache, in whatever way it is implemented:
> truncation of a hugely-mapped file to an unhugely-aligned size would
> easily hit it.
> 
> (Although anon THP could in principle apply khugepaged to private file
> mappings, which are not excluded by the MADV_HUGEPAGE restrictions, in
> practice there's a vm_ops check which excludes them, so it never hits
> this BUG() - there's no interface to "truncate" an anonymous mapping.)
> 
> We could complicate the test, to check i_mmap_rwsem also when there's a
> vm_file; but my inclination was to make zap_pmd_range() more readable by
> simply deleting this check.  A search has shown no report of the issue in
> the years since commit e0897d75f0b2 ("mm, thp: print useful information
> when mmap_sem is unlocked in zap_pmd_range") expanded it from VM_BUG_ON()
> - though I cannot point to what commit I would say then fixed the issue.
> 
> But there are a couple of other patches now floating around, neither
> yet in the tree: let's agree to retain the check as a VM_BUG_ON_VMA(),
> as Matthew Wilcox has done; but subject to a vma_is_anonymous() check,
> as Kirill Shutemov has done.  And let's get this in, without waiting
> for any particular huge pagecache implementation to reach the tree.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov