RE: [PATCH] fs: optimise generic_write_check_limits()

From: David Laight <David.Laight@ACULAB.COM>
To: 'Matthew Wilcox' <willy@infradead.org>,
	Pavel Begunkov <asml.silence@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH] fs: optimise generic_write_check_limits()
Date: Sun, 8 Aug 2021 14:41:13 +0000	[thread overview]
Message-ID: <567d7e15f59a45f6ab94428261b3e473@AcuMS.aculab.com> (raw)
In-Reply-To: <YQ04/NFn8b6cykPQ@casper.infradead.org>

From: Matthew Wilcox
> Sent: 06 August 2021 14:28
> 
> 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.

An option would be to only determine ->s_maxbytes when the size
if larger than MAX_NON_LFS.

So you'd end up with something like:

	if (pos >= max_size) {
		if (!(file->f_flags & O_LARGEFILE))
			return -EFBIG;
		inode = file->f_mapping->host;
		if (pos >= inode->i_sb->s_maxbytes)
			return -EFBIG;
	}

	David

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