linux-kernel - Re: [PATCH 4/6] hugetlbfs: implement memfd sealing

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Date:   Fri, 3 Nov 2017 10:12:59 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Herrmann <dh.herrmann@...il.com>,
        Marc-André Lureau <marcandre.lureau@...hat.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, aarcange@...hat.com,
        Hugh Dickins <hughd@...gle.com>, nyc@...omorphy.com
Subject: Re: [PATCH 4/6] hugetlbfs: implement memfd sealing

On 11/03/2017 10:03 AM, David Herrmann wrote:
> Hi
> 
> On Tue, Oct 31, 2017 at 7:40 PM, Marc-André Lureau
> <marcandre.lureau@...hat.com> wrote:
>> Implements memfd sealing, similar to shmem:
>> - WRITE: deny fallocate(PUNCH_HOLE). mmap() write is denied in
>>   memfd_add_seals(). write() doesn't exist for hugetlbfs.
>> - SHRINK: added similar check as shmem_setattr()
>> - GROW: added similar check as shmem_setattr() & shmem_fallocate()
>>
>> Except write() operation that doesn't exist with hugetlbfs, that
>> should make sealing as close as it can be to shmem support.
> 
> SEAL, SHRINK, and GROW look fine to me.
> 
> Regarding WRITE

The commit message may not be clear.  However, hugetlbfs does not support
the write system call (or aio).  The only way to modify contents of a
hugetlbfs file is via mmap or hole punch/truncate.  So, we do not really
need to worry about those special (a)io cases for hugetlbfs.

-- 
Mike Kravetz

>                 you need to make sure there are no page references
> left around. For instance, on shmem any process might trigger the
> kernel to GUP mapped shmem pages for asynchronous IO, then unmap the
> file and request F_SEAL_WRITE. In this case the seal must be rejected
> *iff* the pages are still pinned. shmem does this by requiring the
> page-refcounts to be 0. Preferably there would be some better
> infrastructure that tells us whether someone operates on those pages,
> but this does not exist right now. See shmem_wait_for_pins() for
> details.
> 
> I have little knowledge on how hugetlbs integrate with the page-cache
> and radix-tree, hence I'd prefer if someone can explicitly ACK that
> shmem_wait_for_pins() is suitable for hugetlbfs.
> 
> Otherwise, this series looks good to me (minus the #ifdef mess..).
> 
> Thanks
> David
> 
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