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

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Date:	Mon, 11 Jun 2012 21:09:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>
Cc:	linuxppc-dev@...ts.ozlabs.org, lkml <linux-kernel@...r.kernel.org>,
	michael@...erman.id.au, antonb@...nktux.localdomain,
	Paul Mackerras <paulus@...ba.org>, benh@...nel.crashing.org,
	Ingo Molnar <mingo@...e.hu>, peterz@...radead.org,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>
Subject: Q: a_ops->readpage() && struct file

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.

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