linux-kernel - Re: [PATCH v7 3.2-rc2 3/30] uprobes: register/unregister probes.

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Date:	Tue, 29 Nov 2011 13:18:07 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>
Subject: Re: [PATCH v7 3.2-rc2 3/30] uprobes: register/unregister probes.

* Peter Zijlstra <peterz@...radead.org> [2011-11-28 16:29:54]:

> On Fri, 2011-11-18 at 16:37 +0530, Srikar Dronamraju wrote:
> > +static void __unregister_uprobe(struct inode *inode, loff_t offset,
> > +                                               struct uprobe *uprobe)
> > +{
> > +       struct list_head try_list;
> > +       struct address_space *mapping;
> > +       struct vma_info *vi, *tmpvi;
> > +       struct vm_area_struct *vma;
> > +       struct mm_struct *mm;
> > +       loff_t vaddr;
> > +
> > +       mapping = inode->i_mapping;
> > +       INIT_LIST_HEAD(&try_list);
> > +       while ((vi = find_next_vma_info(&try_list, offset,
> > +                                               mapping, false)) != NULL) {
> > +               if (IS_ERR(vi))
> > +                       break;
> 
> So what kind of half-assed state are we left in if we try an unregister
> under memory pressure and how do we deal with that?
> 

Agree, Even I had this concern and wanted to see if there are ways to
deal with this.

- One approach would be pass extra GFG flags while we do allocations
  atleast in the unregister_uprobe.

Drawback of this approach: if the system is already under memory
pressure we shouldnt exert more pressure by asking it to repeat.

- The other approach would be to cache these temporary objects while we
  insert probes. i.e keep these metadata around.

I am sure you wouldnt want to add additional metadata.

- Third approach would be to have a completion/worker routine kick in if
  unregister_uprobe fails due to memory allocations.

This looks better than the rest.

Do you have any other approaches that we could try?

-- 
thanks and regards
Srikar

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