Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify hugetlbfs files

From: Miklos Szeredi <miklos@szeredi.hu>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Amir Goldstein <amir73il@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>, Linux MM <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	Colin Walters <walters@verbum.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	syzbot <syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>
Subject: Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify hugetlbfs files
Date: Mon, 15 Jun 2020 09:53:35 +0200	[thread overview]
Message-ID: <CAJfpegsugobr8LnJ7e3D1+QFHCdYkW1swtSZ_hKouf_uhZreMg@mail.gmail.com> (raw)
In-Reply-To: <6e8924b0-bfc4-eaf5-1775-54f506cdf623@oracle.com>

On Sat, Jun 13, 2020 at 9:12 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 6/12/20 11:53 PM, Amir Goldstein wrote:

> As a hugetlbfs developer, I do not know of a use case for interoperability
> with overlayfs.  So yes, I am not too interested in making them work well
> together.  However, if there was an actual use case I would be more than
> happy to consider doing the work.  Just hate to put effort into fixing up
> two 'special' filesystems for functionality that may not be used.
>
> I can't speak for overlayfs developers.

As I said, I only know of tmpfs being upper layer as a valid use case.
   Does that work with hugepages?  How would I go about testing that?

> > I agree with Colin's remark about adding limitations, but it would be a shame
> > if overlay had to special case hugetlbfs. It would have been better if we could
> > find a property of hugetlbfs that makes it inapplicable for overlayfs
> > upper/lower
> > or stacking fs in general.
> >
> > The simplest thing for you to do in order to shush syzbot is what procfs does:
> >         /*
> >          * procfs isn't actually a stacking filesystem; however, there is
> >          * too much magic going on inside it to permit stacking things on
> >          * top of it
> >          */
> >         s->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
> >
> > Currently, the only in-tree stacking fs are overlayfs and ecryptfs, but there
> > are some out of tree implementations as well (shiftfs).
> > So you may only take that option if you do not care about the combination
> > of hugetlbfs with any of the above.
> >
> > overlayfs support of mmap is not as good as one might hope.
> > overlayfs.rst says:
> > "If a file residing on a lower layer is opened for read-only and then
> >  memory mapped with MAP_SHARED, then subsequent changes to
> >  the file are not reflected in the memory mapping."
> >
> > So if I were you, I wouldn't go trying to fix overlayfs-huguetlb interop...
>
> Thanks again,
>
> I'll look at something as simple as s_stack_depth.

Agree.

Thanks,
Miklos