linux-kernel - Re: [PATCH] mm: use only per-device readahead limit

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Message-ID: <CA+55aFy8kOomnL-C5GwSpHTn+g5R7dY78C9=h-J_Rb_u=iASpg@mail.gmail.com>
Date:	Fri, 21 Aug 2015 13:44:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Jan Kara <jack@...e.cz>, Wu Fengguang <fengguang.wu@...el.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: use only per-device readahead limit

On Fri, Aug 21, 2015 at 1:21 PM, Roman Gushchin <klamm@...dex-team.ru> wrote:
>
> It's just a raid driver. For instance, drivers/md/raid5.c:6898 .

Ok. That makes me a bit less nervous. I was worried there was some
admin program out there that just ups the readahead on peoples
devices, which would mean that ra_pages is some random value chosen by
crazy user space people.

> So, I like an idea to delegate the readahead limit calculation to the underlying i/o level.

Yeah, I'm not against it either. It's just that historically we've had
some issues with people over-doing readahead (because it often helps
some made-up microbenchmark), and then we end up with latency issues
when somebody does a multi-gigabyte readahead... Iirc, we had exactly
that problem with the readahead() system call at some point (long
ago).

But if it's just the default ra_pages, then that should be ok. I think
the kernel defaults are generally sane, and I hope there isn't some
crazy distro that ends up mucking with this.

                  Linus
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