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

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Message-Id: <20160119141430.8ff9c464.akpm@linux-foundation.org>
Date:	Tue, 19 Jan 2016 14:14:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Shaohua Li <shli@...com>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH] proc: revert /proc/<pid>/maps [stack:TID] annotation

On Tue, 19 Jan 2016 13:02:39 -0500 Johannes Weiner <hannes@...xchg.org> wrote:

> b764375 ("procfs: mark thread stack correctly in proc/<pid>/maps")
> added [stack:TID] annotation to /proc/<pid>/maps. Finding the task of
> a stack VMA requires walking the entire thread list, turning this into
> quadratic behavior: a thousand threads means a thousand stacks, so the
> rendering of /proc/<pid>/maps needs to look at a million threads. The
> cost is not in proportion to the usefulness as described in the patch.
> 
> Drop the [stack:TID] annotation to make /proc/<pid>/maps (and
> /proc/<pid>/numa_maps) usable again for higher thread counts.
> 
> The [stack] annotation inside /proc/<pid>/task/<tid>/maps is retained,
> as identifying the stack VMA there is an O(1) operation.

Four years ago, ouch.

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.

I note that this patch is a partial revert - the smaps and numa_maps
parts of b764375 remain in place.  What's up with that?