linux-kernel - Re: [PATCH] hugetlbfs: remove call to huge_pte_alloc without i_mmap

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Message-ID: <9072d352-7a07-aac7-3439-3f524fc465ed@oracle.com>
Date:   Mon, 3 Aug 2020 16:00:52 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
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] hugetlbfs: remove call to huge_pte_alloc without
 i_mmap_rwsem

On 8/3/20 3:52 PM, Matthew Wilcox wrote:
> On Mon, Aug 03, 2020 at 03:43:35PM -0700, Mike Kravetz wrote:
>> Commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
>> synchronization") requires callers of huge_pte_alloc to hold i_mmap_rwsem
>> in at least read mode.  This is because the explicit locking in
>> huge_pmd_share (called by huge_pte_alloc) was removed.  When restructuring
>> the code, the call to huge_pte_alloc in the else block at the beginning
>> of hugetlb_fault was missed.
> 
> Should we have a call to mmap_assert_locked() in huge_pte_alloc(),
> at least the generic one?

That is the wrong semaphore.

However, I was not aware of the checks for a semaphore being held as is
done in rwsem_is_locked().  That would have caught this when the original
code was changed.  Thanks for pointing this out.

Let me update the patch and add checks to huge_pmd_share().
-- 
Mike Kravetz