linux-kernel - Re: [PATCH 1/3] uprobes: install_breakpoint() should fail if is_swbp

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Date:	Fri, 1 Jun 2012 22:03:15 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] uprobes: install_breakpoint() should fail if
 is_swbp_insn() == T

* Oleg Nesterov <oleg@...hat.com> [2012-06-01 17:53:12]:

> On 05/31, Oleg Nesterov wrote:
> >
> > __find_next_vma_info() checks tmpvi->mm == vma->vm_mm to detect the
> > already visited mm/vma. However, afaics this can be false positive?
> 
> Yes, but I guess this is harmless, we can rely on uprobe_mmap.
> 
> 
> 
> But. Doesn't this mean we can greatly simplify register_for_each_vma()
> and make it O(n) ?
> 
> Unless I missed something, we can simply create the list of
> mm/vaddr structures under ->i_mmap_mutex (vma_prio_tree_foreach), then
> register_for_each_vma() can process the list and that is all.


If I remember correctly, we cannot allocate the list elements under
i_mmap_mutex. We dont know how many list elements to allocate.

This is what Peter had to say : https://lkml.org/lkml/2011/6/27/72

"Because we try to take i_mmap_mutex during reclaim, trying to unmap
pages. So suppose we do an allocation while holding i_mmap_mutex, find
there's no free memory, try and unmap a page in order to free it, and
we're stuck."

> 
> If another mapping comes after we drop i_mmap_mutex, uprobe_mmap()
> should be called and it should install the bp.
> 

-- 
Thanks and Regards
Srikar

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