Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

[Eric Dumazet]

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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