linux-kernel - Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped

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Date:	Wed, 14 Jan 2015 15:13:44 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Calvin Owens <calvinowens@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Feiner <pfeiner@...gle.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>,
	<linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped_files

On Wed, Jan 14 2015, Calvin Owens <calvinowens@...com> wrote:

> Commit b76437579d1344b6 ("procfs: mark thread stack correctly in
> proc/<pid>/maps") introduced logic to mark thread stacks with the
> "[stack:%d]" marker in /proc/<pid>/maps.
>
> This causes reading /proc/<pid>/maps to take O(N^2) time, where N is
> the number of threads sharing an address space, since each line of
> output requires iterating over the VMA list looking for ranges that
> correspond to the stack pointer in any task's register set. When
> dealing with highly-threaded Java applications, reading this file can
> take hours and trigger softlockup dumps.
>
> Eliminating the "[stack:%d]" marker is not a viable option since it's
> been there for some time, and I don't see a way to do the stack check
> more efficiently that wouldn't end up making the whole thing really
> ugly.

Just thinking out loud: Could one simply mark a VMA as being used for
stack during the clone call (is there room in vm_flags, or does
VM_GROWSDOWN already tell the whole story?), and then write the TID into
a new field in the VMA - I think one could make a union with vm_pgoff so
as not to enlarge the structure.

This would allow eliminating the loop over tasks in vm_is_stack.

Rasmus
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