Q: a_ops->readpage() && struct file

From: Oleg Nesterov <oleg@redhat.com>
To: Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@ZenIV.linux.org.uk>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	peterz@infradead.org, antonb@thinktux.localdomain,
	lkml <linux-kernel@vger.kernel.org>,
	michael@ellerman.id.au, Jim Keniston <jkenisto@us.ibm.com>,
	Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@elte.hu>,
	linuxppc-dev@lists.ozlabs.org
Subject: Q: a_ops->readpage() && struct file
Date: Mon, 11 Jun 2012 21:09:02 +0200	[thread overview]
Message-ID: <20120611190902.GA19869@redhat.com> (raw)
In-Reply-To: <20120611161215.GA12116@redhat.com>

On 06/11, Oleg Nesterov wrote:
>
> Note also that we should move this !UPROBE_COPY_INSN from
> install_breakpoint() to somewhere near alloc_uprobe().

The main problem is, uprobe_register() doesn't have struct file
for read_mapping_page().

Stupid question. I'm afraid the answer is "no" but I'll ask anyway.
Is it safe to pass filp == NULL to mapping->readpage()? In fact
I do not understand why it needs "struct file*" and I do not see
any example of actual usage. Yes, I didn't try to grep very much
and I understand that the filesystem can do something special.
Say it can use file->private_data...

However. There is read_cache_page_gfp() which does use
a_ops->readpage(filp => NULL), and the comment says nothing about
the riskiness.

If not, is there any other way
uprobe_register(struct inode *inode, loff_t offset) can read the
page at this offset?

And btw, read_mapping_page() accepts "void *data". Why? it uses
filler == a_ops->readpage, it shouldn't accept anything but file
pointer?

Please help, I know nothing about vfs.

Oleg.