From: Dave Hansen <dave.hansen@intel.com>
To: Daniel Colascione <dancol@google.com>
Cc: linux-mm@kvack.org, Tim Murray <timmurray@google.com>,
Minchan Kim <minchan@kernel.org>
Subject: Re: Why do we let munmap fail?
Date: Mon, 21 May 2018 16:32:10 -0700 [thread overview]
Message-ID: <20c9acc2-fbaf-f02d-19d7-2498f875e4c0@intel.com> (raw)
In-Reply-To: <CAKOZueu8ckN1b-cYOxPhL5f7Bdq+LLRP20NK3x7Vtw79oUT3pg@mail.gmail.com>
On 05/21/2018 04:16 PM, Daniel Colascione wrote:
> On Mon, May 21, 2018 at 4:02 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
>> On 05/21/2018 03:54 PM, Daniel Colascione wrote:
>>>> There are also certainly denial-of-service concerns if you allow
>>>> arbitrary numbers of VMAs. The rbtree, for instance, is O(log(n)), but
>>>> I 'd be willing to be there are plenty of things that fall over if you
>>>> let the ~65k limit get 10x or 100x larger.
>>> Sure. I'm receptive to the idea of having *some* VMA limit. I just think
>>> it's unacceptable let deallocation routines fail.
>> If you have a resource limit and deallocation consumes resources, you
>> *eventually* have to fail a deallocation. Right?
> That's why robust software sets aside at allocation time whatever resources
> are needed to make forward progress at deallocation time.
I think there's still a potential dead-end here. "Deallocation" does
not always free resources.
> That's what I'm trying to propose here, essentially: if we specify
> the VMA limit in terms of pages and not the number of VMAs, we've
> effectively "budgeted" for the worst case of VMA splitting, since in
> the worst case, you end up with one page per VMA.
Not a bad idea, but it's not really how we allocate VMAs today. You
would somehow need per-process (mm?) slabs. Such a scheme would
probably, on average, waste half of a page per mm.
> Done this way, we still prevent runaway VMA tree growth, but we can also
> make sure that anyone who's successfully called mmap can successfully call
> munmap.
I'd be curious how this works out, but I bet you end up reserving a lot
more resources than people want.
next prev parent reply other threads:[~2018-05-21 23:32 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-21 22:07 Why do we let munmap fail? Daniel Colascione
2018-05-21 22:12 ` Dave Hansen
2018-05-21 22:20 ` Daniel Colascione
2018-05-21 22:29 ` Dave Hansen
2018-05-21 22:35 ` Daniel Colascione
2018-05-21 22:48 ` Dave Hansen
2018-05-21 22:54 ` Daniel Colascione
2018-05-21 23:02 ` Dave Hansen
2018-05-21 23:16 ` Daniel Colascione
2018-05-21 23:32 ` Dave Hansen [this message]
2018-05-22 0:00 ` Daniel Colascione
2018-05-22 0:22 ` Matthew Wilcox
2018-05-22 0:38 ` Daniel Colascione
2018-05-22 1:19 ` Theodore Y. Ts'o
2018-05-22 1:41 ` Daniel Colascione
2018-05-22 2:09 ` Daniel Colascione
2018-05-22 2:11 ` Matthew Wilcox
2018-05-22 1:22 ` Matthew Wilcox
2018-05-22 5:34 ` Nicholas Piggin
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