netdev - Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)

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Date:   Sun, 25 Sep 2016 11:50:05 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     netdev@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)

Eric Dumazet <eric.dumazet@...il.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