linux-kernel - Re: [PATCH] proc: revert /proc/<pid>/maps [stack:TID] annotation

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Message-Id: <20160119212745.eee310f5.akpm@linux-foundation.org>
Date:	Tue, 19 Jan 2016 21:27:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, Shaohua Li <shli@...com>,
	linux-mm@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
	kernel-team@...com
Subject: Re: [PATCH] proc: revert /proc/<pid>/maps [stack:TID] annotation

On Wed, 20 Jan 2016 08:47:39 +0530 Siddhesh Poyarekar <siddhesh.poyarekar@...il.com> wrote:

> On 20 January 2016 at 03:44, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > Any thoughts on the obvious back-compatibility concerns?  ie, why did
> > Siddhesh implement this in the first place?  My bad for not ensuring
> > that the changelog told us this.
> >
> > https://lkml.org/lkml/2012/1/14/25 has more info:
> >
> > : Memory mmaped by glibc for a thread stack currently shows up as a
> > : simple anonymous map, which makes it difficult to differentiate between
> > : memory usage of the thread on stack and other dynamic allocation.
> > : Since glibc already uses MAP_STACK to request this mapping, the
> > : attached patch uses this flag to add additional VM_STACK_FLAGS to the
> > : resulting vma so that the mapping is treated as a stack and not any
> > : regular anonymous mapping.  Also, one may use vm_flags to decide if a
> > : vma is a stack.
> >
> > But even that doesn't really tell us what the actual *value* of the
> > patch is to end-users.
> 
> The end users needed a way to identify thread stacks programmatically
> and there wasn't a way to do that.  I'm afraid I no longer remember
> (or have access to the resources that would aid my memory since I
> changed employers) the details of their requirement.  However, I did
> do this on my own time because I thought it was an interesting project
> for me and nobody really gave any feedback then as to its utility, so
> as far as I am concerned you could roll back the main thread maps
> information since the information is available in the thread-specific
> files.

OK, thanks.  I was thinking of queueing this for 4.6 to let it bake in
-next for a cycle, but quadratic performance is bad and nobody will
test such an obscure feature in -next so maybe I'll jam it into 4.5 and we
wait and see.