linux-kernel - Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

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Message-ID: <20110614142023.GA5139@redhat.com>
Date:	Tue, 14 Jun 2011 16:20:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 2/22]  2: uprobes: Breakground page
	replacement.

On 06/14, Srikar Dronamraju wrote:
>
> > > +static int write_opcode(struct task_struct *tsk, struct uprobe * uprobe,
> > > +			unsigned long vaddr, uprobe_opcode_t opcode)
> > > +{
> > > +	struct page *old_page, *new_page;
> > > +	void *vaddr_old, *vaddr_new;
> > > +	struct vm_area_struct *vma;
> > > +	unsigned long addr;
> > > +	int ret;
> > > +
> > > +	/* Read the page with vaddr into memory */
> > > +	ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma);
> >
> > Sorry if this was already discussed... But why we are using FOLL_WRITE here?
> > We are not going to write into this page, and this provokes the unnecessary
> > cow, no?
>
> Yes, We are not going to write to the page returned by get_user_pages
> but a copy of that page.

Yes I see. But the page returned by get_user_pages(write => 1) is already
a cow'ed copy (this mapping should be read-only).

> The idea was if we cow the page then we dont
> need to cow it at the replace_page time

Yes, replace_page() shouldn't cow.

> and since get_user_pages knows
> the right way to cow the page, we dont have to write another routine to
> cow the page.

Confused. write_opcode() allocs another page and does memcpy. This is
correct, but I don't understand the first cow.

> I am still not clear on your concern.

Probably I missed something... but could you please explain why we can't

	- ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma);
	+ ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 0, 0, &old_page, &vma);

?

> > Also. This is called under down_read(mmap_sem), can't we race with
> > access_process_vm() modifying the same memory?
>
> Yes, we could be racing with access_process_vm on the same memory.
>
> Do we have any other option other than making write_opcode/read_opcode
> being called under down_write(mmap_sem)?

I dunno. Probably we can simply ignore this issue, there are other ways
to modify this memory.

Oleg.

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