Re: [PATCH -V2] hugetlbfs: Drop taking inode i_mutex lock from hugetlbfs_read

[Josh Boyer <jwboyer@gmail.com>]

On Thu, Mar 1, 2012 at 5:10 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Thu,  1 Mar 2012 14:48:50 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
>
>> Taking i_mutex lock in hugetlbfs_read can result in deadlock with mmap
>> as explained below
>>  Thread A:
>>   read() on hugetlbfs
>>    hugetlbfs_read() called
>>     i_mutex grabbed
>>      hugetlbfs_read_actor() called
>>       __copy_to_user() called
>>        page fault is triggered
>>  Thread B, sharing address space with A:
>>   mmap() the same file
>>    ->mmap_sem is grabbed on task_B->mm->mmap_sem
>>     hugetlbfs_file_mmap() is called
>>      attempt to grab ->i_mutex and block waiting for A to give it up
>>  Thread A:
>>   pagefault handled blocked on attempt to grab task_A->mm->mmap_sem,
>>  which happens to be the same thing as task_B->mm->mmap_sem.  Block waiting
>>  for B to give it up.
>>
>> AFAIU i_mutex lock got added to  hugetlbfs_read as per
>> http://lkml.indiana.edu/hypermail/linux/kernel/0707.2/3066.html
>> to take care of the race between truncate and read. This patch fix
>> this by looking at page->mapping under page_lock (find_lock_page())
>> to ensure; the inode didn't get truncated in the range during a
>> parallel read.
>>
>> Ideally we can extend the patch to make sure we don't increase i_size
>> in mmap. But that will break userspace, because application will now
>> have to use truncate(2) to increase i_size in hugetlbfs.
>
> Looks OK to me.
>
> Given that the bug has been there for four years, I'm assuming that
> we'll be OK merging this fix into 3.4.  Or we could merge it into 3.4
> and tag it for backporting into earlier kernels - it depends on whether
> people are hurting from it, which I don't know?

We've gotten a few lockdep reports about it in Fedora on various kernels.
A CC to stable might be nice.

josh

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