From: Andi Kleen <andi@firstfloor.org>
To: linux-mm@kvack.org
Cc: linux-fsdevel@vger.kernel.org,linux-kernel@vger.kernel.org,netdev@vger.kernel.org
Subject: Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)
Date: Sun, 25 Sep 2016 11:50:05 -0700 [thread overview]
Message-ID: <87r387oluq.fsf@tassilo.jf.intel.com> (raw)
In-Reply-To: 1474562982.23058.140.camel@edumazet-glaptop3.roam.corp.google.com
Eric Dumazet <eric.dumazet@gmail.com> writes:
> On Thu, 2016-09-22 at 18:43 +0200, Vlastimil Babka wrote:
>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
>> with the number of fds passed. We had a customer report page allocation
>> failures of order-4 for this allocation. This is a costly order, so it might
>> easily fail, as the VM expects such allocation to have a lower-order fallback.
>>
>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
>> physically contiguous. Also the allocation is temporary for the duration of the
>> syscall, so it's unlikely to stress vmalloc too much.
>
> vmalloc() uses a vmap_area_lock spinlock, and TLB flushes.
>
> So I guess allowing vmalloc() being called from an innocent application
> doing a select() might be dangerous, especially if this select() happens
> thousands of time per second.
Yes it seems like a bad idea because of all the scaling problems here.
The right solution would be to fix select to use multiple
non virtually contiguous pages.
-Andi
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next prev parent reply other threads:[~2016-09-25 18:50 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-22 16:43 [PATCH v2] fs/select: add vmalloc fallback for select(2) Vlastimil Babka
2016-09-22 16:49 ` Eric Dumazet
2016-09-22 16:56 ` Vlastimil Babka
2016-09-22 17:07 ` Eric Dumazet
2016-09-22 17:55 ` Vlastimil Babka
2016-09-23 9:42 ` David Laight
2016-09-23 9:58 ` Vlastimil Babka
2016-09-23 13:35 ` David Laight
2016-09-26 10:01 ` Vlastimil Babka
2016-09-26 15:02 ` David Laight
2016-09-25 18:50 ` Andi Kleen [this message]
2016-09-27 0:01 ` Andrew Morton
2016-09-27 1:38 ` Eric Dumazet
2016-09-27 8:13 ` Vlastimil Babka
2016-09-27 13:34 ` Eric Dumazet
2016-09-27 8:06 ` Vlastimil Babka
2016-09-27 8:45 ` [PATCH v3] " Vlastimil Babka
2016-09-27 10:22 ` Michal Hocko
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