linux-kernel - Re: [PATCH 1/2] Revert "vmalloc: back off when the current task is killed"

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Date:   Wed, 4 Oct 2017 15:32:45 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Alan Cox <alan@...yncelyn.cymru>, Christoph Hellwig <hch@....de>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/2] Revert
 "vmalloc: back off when the current task is killed"

On Wed, 4 Oct 2017 14:59:06 -0400 Johannes Weiner <hannes@...xchg.org> wrote:

> This reverts commit 5d17a73a2ebeb8d1c6924b91e53ab2650fe86ffb and
> commit 171012f561274784160f666f8398af8b42216e1f.
> 
> 5d17a73a2ebe ("vmalloc: back off when the current task is killed")
> made all vmalloc allocations from a signal-killed task fail. We have
> seen crashes in the tty driver from this, where a killed task exiting
> tries to switch back to N_TTY, fails n_tty_open because of the vmalloc
> failing, and later crashes when dereferencing tty->disc_data.
> 
> Arguably, relying on a vmalloc() call to succeed in order to properly
> exit a task is not the most robust way of doing things. There will be
> a follow-up patch to the tty code to fall back to the N_NULL ldisc.
> 
> But the justification to make that vmalloc() call fail like this isn't
> convincing, either. The patch mentions an OOM victim exhausting the
> memory reserves and thus deadlocking the machine. But the OOM killer
> is only one, improbable source of fatal signals. It doesn't make sense
> to fail allocations preemptively with plenty of memory in most cases.
> 
> The patch doesn't mention real-life instances where vmalloc sites
> would exhaust memory, which makes it sound more like a theoretical
> issue to begin with. But just in case, the OOM access to memory
> reserves has been restricted on the allocator side in cd04ae1e2dc8
> ("mm, oom: do not rely on TIF_MEMDIE for memory reserves access"),
> which should take care of any theoretical concerns on that front.
> 
> Revert this patch, and the follow-up that suppresses the allocation
> warnings when we fail the allocations due to a signal.

You don't think they should be backported into -stables?