linux-kernel - Re: [PATCH] fs: optimise generic_write_check

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Message-ID: <YQ04/NFn8b6cykPQ@casper.infradead.org>
Date:   Fri, 6 Aug 2021 14:28:28 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: optimise generic_write_check_limits()

On Fri, Aug 06, 2021 at 12:22:10PM +0100, Pavel Begunkov wrote:
> Even though ->s_maxbytes is used by generic_write_check_limits() only in
> case of O_LARGEFILE, the value is loaded unconditionally, which is heavy
> and takes 4 indirect loads. Optimise it by not touching ->s_maxbytes,
> if it's not going to be used.

Is this "optimisation" actually worth anything?  Look at how
force_o_largefile() is used.  I would suggest that on the vast majority
of machines, O_LARGEFILE is always set.