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

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Message-ID: <6e62a278-4ac3-a866-51c6-e32511406aba@suse.cz>
Date:   Tue, 27 Sep 2016 10:13:16 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
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
Subject: Re: [PATCH v2] fs/select: add vmalloc fallback for select(2)

On 09/27/2016 03:38 AM, Eric Dumazet wrote:
> On Mon, 2016-09-26 at 17:01 -0700, Andrew Morton wrote:
>
>> I don't share Eric's concerns about performance here.  If the vmalloc()
>> is called, we're about to write to that quite large amount of memory
>> which we just allocated, and the vmalloc() overhead will be relatively
>> low.
>
> I did not care of the performance of this particular select() system
> call really, but other cpus because of more TLB invalidations.

There are many other ways to cause those, AFAIK. The reclaim/compaction
for order-3 allocation has its own impact on system, including TLB flushes.
Or a flood of mmap(MAP_POPULATE) and madvise(MADV_DONTNEED) calls...
This vmalloc() would however require raising RLIMIT_NOFILE above the defaults.

> At least CONFIG_DEBUG_PAGEALLOC=y builds should be impacted, but maybe
> we do not care.

I doubt anyone runs that in production, especially if performance is of concern.