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

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Date:	Wed, 20 Jan 2016 01:30:28 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>, 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, Jan 19, 2016 at 02:14:30PM -0800, Andrew Morton wrote:
> 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 doubt it can be very useful as it's unreliable: if two stacks are
allocated end-to-end (which is not good idea, but still) it can only
report [stack:XXX] for the first one as they are merged into one VMA.
Any other anon VMA merged with the stack will be also claimed as stack,
which is not always correct.

I think report the VMA as anon is the best we can know about it,
everything else just rather expensive guesses.

> 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?
> 
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-- 
 Kirill A. Shutemov