linux-kernel - Re: [RFC PATCH 0/4 v2] Define killable version for access_remote

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Date:   Mon, 26 Feb 2018 17:47:47 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
cc:     akpm@...ux-foundation.org, mingo@...nel.org, adobriyan@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm()
 and use it in fs/proc

On Mon, 26 Feb 2018, Yang Shi wrote:

> > Rather than killable, we have patches that introduce down_read_unfair()
> > variants for the files you've modified (cmdline and environ) as well as
> > others (maps, numa_maps, smaps).
> 
> You mean you have such functionality used by google internally?
> 

Yup, see https://lwn.net/Articles/387720.

> > When another thread is holding down_read() and there are queued
> > down_write()'s, down_read_unfair() allows for grabbing the rwsem without
> > queueing for it.  Additionally, when another thread is holding
> > down_write(), down_read_unfair() allows for queueing in front of other
> > threads trying to grab it for write as well.
> 
> It sounds the __unfair variant make the caller have chance to jump the gun to
> grab the semaphore before other waiters, right? But when a process holds the
> semaphore, i.e. mmap_sem, for a long time, it still has to sleep in
> uninterruptible state, right?
> 

Right, it's solving two separate things which I think may be able to be 
merged together.  Killable is solving an issue where the rwsem is blocking 
for a long period of time in uninterruptible sleep, and unfair is solving 
an issue where reading the procfs files gets stalled for a long period of 
time.  We haven't run into an issue (yet) where killable would have solved 
it; we just have the unfair variants to grab the rwsem asap and then, if 
killable, gracefully return.

> > Ingo would know more about whether a variant like that in upstream Linux
> > would be acceptable.
> > 
> > Would you be interested in unfair variants instead of only addressing
> > killable?
> 
> Yes, I'm although it still looks overkilling to me for reading /proc.
> 

We make certain inferences on the readablility of procfs files for other 
threads to determine how much its mm's mmap_sem is contended.