linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Date:	Mon, 20 Apr 2015 10:15:26 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

On Mon, 2015-04-20 at 17:10 +0200, Mateusz Guzik wrote:

> Sorry for spam but I came up with another hack. :)
> 
> The idea is that we can have a variable which would signify the that
> given thread is playing with fd table in fd_install (kind of a lock
> embedded into task_struct). We would also have a flag in files struct
> indicating that a thread would like to resize it.
> 
> expand_fdtable would set the flag and iterate over all threads waiting
> for all of them to have the var set to 0.

The opposite : you have to block them in some way and add a rcu_sched()
or something.

Another way would be to expand the table leaving the old one in place,
thanks to a new vrealloc() api. This would require all file tables to at
least use one page.

(Instead of use a new set of pages for the new vmalloc()ed area, reuse
the pages that are already mapped into previous vmalloc() area.

Right now, expanding 4M slots to 8M slots is taking a long time and is a
latency killer.

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