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

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, Jun 16, 2020 at 1:45 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 6/15/20 12:53 AM, Miklos Szeredi wrote:
> > 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:
> >>>
> >>> 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.
>
> Apologies again for in the incorrect information about writing to lower
> filesystem.
>
> Stacking ecryptfs on hugetlbfs does not work either.  Here is what happens
> when trying to create a new file.
>
> [ 1188.863425] ecryptfs_write_metadata_to_contents: Error attempting to write header information to lower file; rc = [-22]
> [ 1188.865469] ecryptfs_write_metadata: Error writing metadata out to lower file; rc = [-22]
> [ 1188.867022] Error writing headers; rc = [-22]
>
> I like Amir's idea of just setting s_stack_depth in hugetlbfs to prevent
> stacking.
>
> From 0fbed66b37c18919ea7edd47b113c97644f49362 Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <mike.kravetz@oracle.com>
> Date: Mon, 15 Jun 2020 14:37:52 -0700
> Subject: [PATCH] hugetlbfs: prevent filesystem stacking of hugetlbfs
>
> syzbot found issues with having hugetlbfs on a union/overlay as reported
> in [1].  Due to the limitations (no write) and special functionality of
> hugetlbfs, it does not work well in filesystem stacking.  There are no
> know use cases for hugetlbfs stacking.  Rather than making modifications
> to get hugetlbfs working in such environments, simply prevent stacking.
>
> [1] https://lore.kernel.org/linux-mm/000000000000b4684e05a2968ca6@google.com/
>
> Reported-by: syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com
> Suggested-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Thanks,
Miklos