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From: Mike Kravetz <mike.kravetz@oracle.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: 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>,
	Miklos Szeredi <miklos@szeredi.hu>,
	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: Sat, 13 Jun 2020 12:12:04 -0700	[thread overview]
Message-ID: <6e8924b0-bfc4-eaf5-1775-54f506cdf623@oracle.com> (raw)
In-Reply-To: <CAOQ4uxg=o2SVbfUiz0nOg-XHG8irvAsnXzFWjExjubk2v_6c_A@mail.gmail.com>

On 6/12/20 11:53 PM, Amir Goldstein wrote:
>>> Incidentally, can a hugetlbfs be a lower layer, while the upper one
>>> is a normal filesystem?  What should happen on copyup?
>>
>> Yes, that seems to work as expected.  When accessed for write the hugetlb
>> file is copied to the normal filesystem.
>>
>> The BUG found by syzbot actually has a single hugetlbfs as both lower and
>> upper.  With the BUG 'fixed', I am not exactly sure what the expected
>> behavior is in this case.  I may be wrong, but I would expect any operations
>> that can be performed on a stand alone hugetlbfs to also be performed on
>> the overlay.  However, mmap() still fails.  I will look into it.
>>
>> I also looked at normal filesystem lower and hugetlbfs upper.  Yes, overlayfs
>> allows this.  This is somewhat 'interesting' as write() is not supported in
>> hugetlbfs.  Writing to files in the overlay actually ended up writing to
>> files in the lower filesystem.  That seems wrong, but overlayfs is new to me.
>>
> 
> I am not sure how that happened, but I think that ovl_open_realfile()
> needs to fixup f_mode flags FMODE_CAN_WRITE | FMODE_CAN_READ
> after open_with_fake_path().
> 
>> Earlier in the discussion of these issues, Colin Walters asked "Is there any
>> actual valid use case for mounting an overlayfs on top of hugetlbfs?"  I can
>> not think of one.  Perhaps we should consider limiting the ways in which
>> hugetlbfs can be used in overlayfs?  Preventing it from being an upper
>> filesystem might be a good start?  Or, do people think making hugetlbfs and
>> overlayfs play nice together is useful?
> 
> If people think that making hugetlbfs and overlayfs play nice together maybe
> they should work on this problem. It doesn't look like either
> hugetlbfs developers
> nor overlayfs developers care much about the combination.

Thanks Amir,

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.

> Your concern, I assume, is fixing the syzbot issue.

That is the primary concern.  We should not BUG!  After fixing that up, Al
asked how these things worked together.  I honestly did not look at
interoperability before that.  I am not sure if anyone has done that in the
past.

> 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.
-- 
Mike Kravetz

  parent reply	other threads:[~2020-06-13 19:12 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-12  0:46 Mike Kravetz
2020-06-12  0:46 ` [PATCH v4 2/2] ovl: call underlying get_unmapped_area() routine. propogate FMODE_HUGETLBFS Mike Kravetz
2020-06-14 12:50   ` Amir Goldstein
2020-06-12  1:53 ` [PATCH v4 1/2] hugetlb: use f_mode & FMODE_HUGETLBFS to identify hugetlbfs files Matthew Wilcox
2020-06-12  1:58 ` Al Viro
2020-06-12 21:51   ` Mike Kravetz
2020-06-13  6:53     ` Amir Goldstein
2020-06-13 14:38       ` Matthew Wilcox
2020-06-13 19:12       ` Mike Kravetz [this message]
2020-06-15  7:53         ` Miklos Szeredi
2020-06-15 10:05           ` Amir Goldstein
2020-06-15 13:01             ` Miklos Szeredi
2020-06-15 23:45           ` Mike Kravetz
2020-06-16  9:01             ` Miklos Szeredi
2020-06-15  8:24       ` Miklos Szeredi
2020-06-15 17:48         ` Mike Kravetz
2020-06-12  6:28 ` [RFC PATCH] hugetlb: hugetlbfs_file_operations can be static kernel test robot

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