linux-kernel - Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE

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Date:   Mon, 15 Jun 2020 09:53:35 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Colin Walters <walters@...bum.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@...kaller.appspotmail.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify
 hugetlbfs files

On Sat, Jun 13, 2020 at 9:12 PM Mike Kravetz <mike.kravetz@...cle.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