Re: [PATCH 01/12] uprobes: split THPs before trying replace them

From: Rik van Riel <riel@redhat.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Hugh Dickins <hughd@google.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 01/12] uprobes: split THPs before trying replace them
Date: Wed, 25 Jan 2017 12:44:29 -0500	[thread overview]
Message-ID: <1485366269.29861.0.camel@redhat.com> (raw)
In-Reply-To: <20170125165522.GA11569@linux.vnet.ibm.com>

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On Wed, 2017-01-25 at 08:55 -0800, Srikar Dronamraju wrote:
> > 
> > > 
> > > 
> > > > 
> > > > For THPs page_check_address() always fails. It's better to
> > > > split them
> > > > first before trying to replace.
> > > So what does this mean.  uprobes simply fails to work when trying
> > > to
> > > place a probe into a THP memory region?
> > Looks like we can end up with endless retry loop in
> > uprobe_write_opcode().
> > 
> > > 
> > > How come nobody noticed (and reported) this when using the
> > > feature?
> > I guess it's not often used for anon memory.
> > 
> The first time the breakpoint is hit on a page, it replaces the text
> page with anon page.  Now lets assume we insert breakpoints in all
> the
> pages in a range. Here each page is individually replaced by a non
> THP
> anonpage. (since we dont have bulk breakpoint insertion support,
> breakpoint insertion happens one at a time). Now the only interesting
> case may be when each of these replaced pages happen to be physically
> contiguous so that THP kicks in to replace all of these pages with
> one
> THP page. Can happen in practice?
> 
> Are there any other cases that I have missed?

A JIT compiler placing executable code in anonymous
memory before executing it, and a debugger trying to
insert a uprobe in one of those areas?

Not common, but I suppose it could be done.

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