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

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Date:   Tue, 27 Sep 2016 10:44:04 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Jason Baron <jbaron@...mai.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Hillf Danton <hillf.zj@...baba-inc.com>
Cc:     'Alexander Viro' <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, 'Michal Hocko' <mhocko@...nel.org>,
        netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] fs/select: add vmalloc fallback for select(2)

On 09/23/2016 06:47 PM, Jason Baron wrote:
> Hi,
>
> On 09/23/2016 03:24 AM, Nicholas Piggin wrote:
>> On Fri, 23 Sep 2016 14:42:53 +0800
>> "Hillf Danton" <hillf.zj@...baba-inc.com> 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.
>>>>
>>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
>>>> it doesn't need this kind of fallback.
>>
>> How about something like this? (untested)

This pushes the limit further, but might just delay the problem. Could be an 
optimization on top if there's enough interest, though.

[...]

>> +
>> +		if (!(fds.in && fds.out && fds.ex &&
>> +				fds.res_in && fds.res_out && fds.res_ex))
>> +			goto out;
>> +	} else {
>> +		if (nr_bytes > sizeof(stack_fds)) {
>> +			/* Not enough space in on-stack array */
>> +			if (nr_bytes > PAGE_SIZE * 2)
>
> The 'if' looks extraneous?
>
> Also, I wonder if we can just avoid some allocations altogether by
> checking by if the user fd_set pointers are NULL? That can avoid failures :)

That would be a more major rewrite, as the core algorithm doesn't expect NULLs.

> Thanks,
>
> -Jason
>