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From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Calvin Owens <calvinowens@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
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        Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>,
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Subject: Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped_files
Organization: D03
References: <1421194829-28696-1-git-send-email-calvinowens@fb.com>
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Date: Wed, 14 Jan 2015 15:13:44 +0100
In-Reply-To: <1421194829-28696-1-git-send-email-calvinowens@fb.com> (Calvin
	Owens's message of "Tue, 13 Jan 2015 16:20:29 -0800")
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On Wed, Jan 14 2015, Calvin Owens <calvinowens@fb.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