linux-kernel - Re: [PATCH 0/2] fs: make the i_size_read/write helpers be smp_load_acquire/store

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Message-ID: <20240123185622.ssscyrrw5mjqjdyh@quack3>
Date: Tue, 23 Jan 2024 19:56:22 +0100
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: Baokun Li <libaokun1@...wei.com>, torvalds@...ux-foundation.org,
	viro@...iv.linux.org.uk, jack@...e.cz, willy@...radead.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com, yukuai3@...wei.com,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/2] fs: make the i_size_read/write helpers be
 smp_load_acquire/store_release()

On Mon 22-01-24 12:14:52, Christian Brauner wrote:
> On Mon, 22 Jan 2024 17:45:34 +0800, Baokun Li wrote:
> > This patchset follows the linus suggestion to make the i_size_read/write
> > helpers be smp_load_acquire/store_release(), after which the extra smp_rmb
> > in filemap_read() is no longer needed, so it is removed.
> > 
> > Functional tests were performed and no new problems were found.
> > 
> > Here are the results of unixbench tests based on 6.7.0-next-20240118 on
> > arm64, with some degradation in single-threading and some optimization in
> > multi-threading, but overall the impact is not significant.
> > 
> > [...]
> 
> Hm, we can certainly try but I wouldn't rule it out that someone will
> complain aobut the "non-significant" degradation in single-threading.
> We'll see. Let that performance bot chew on it for a bit as well.

Yeah, over 5% regression in buffered read/write cost is a bit hard to
swallow. I somewhat wonder why this is so much - maybe people call
i_size_read() without thinking too much and now it becomes atomic op on
arm? Also LKP tests only on x86 (where these changes are going to be
for noop) and I'm not sure anybody else runs performance tests on
linux-next, even less so on ARM... So not sure anybody will complain until
this gets into some distro (such as Android).

> But I agree that the smp_load_acquire()/smp_store_release() is clearer
> than the open-coded smp_rmb().

Agreed, conceptually this is nice and it will also silence some KCSAN
warnings about i_size updates vs reads.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR