linux-kernel - Re: [PATCH v2] ovl: provide real_file() and overlayfs get_unmapped

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Message-ID: <20200611003726.GY23230@ZenIV.linux.org.uk>
Date:   Thu, 11 Jun 2020 01:37:26 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        kbuild test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        Colin Walters <walters@...bum.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        overlayfs <linux-unionfs@...r.kernel.org>
Subject: Re: [PATCH v2] ovl: provide real_file() and overlayfs
 get_unmapped_area()

On Wed, Jun 10, 2020 at 05:13:52PM -0700, Mike Kravetz wrote:

> To address this issue,
> - Add a new file operation f_real while will return the underlying file.
>   Only overlayfs provides a function for this operation.
> - Add a new routine real_file() which can be used by core code get an
>   underlying file.
> - Update is_file_hugepages to get the real file.

Egads...  So to find out whether it's a hugetlb you would
	* check if a method is NULL
	* if not, call it
	* ... and check if the method table of the result is hugetlbfs one?

Here's a radical suggestion: FMODE_HUGEPAGES.  Just have it set by
->open() and let is_file_hugepages() check it.  In ->f_mode.  And
make the bloody hugetlbfs_file_operations static, while we are at it.