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

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Message-ID: <20200611013616.GM19604@bombadil.infradead.org>
Date:   Wed, 10 Jun 2020 18:36:16 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        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 Thu, Jun 11, 2020 at 01:37:26AM +0100, Al Viro wrote:
> 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.

ITYM FMODE_OVL_UPPER.  To quote Mike:

>         while (file->f_op == &ovl_file_operations)
>                 file = file->private_data;
>         return file;

which would be transformed into:

	while (file->f_mode & FMODE_OVL_UPPER)
		file = file->private_data;
	return file;

Or are you proposing that overlayfs copy FMODE_HUGEPAGES from the
underlying fs to the overlaying fs?