linux-kernel - Re: [PATCH 2/2] fs: generic_file_buffered_read() now uses find_get_pages

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Message-ID: <20200610010800.GA4070152@moria.home.lan>
Date:   Tue, 9 Jun 2020 21:08:00 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: generic_file_buffered_read() now uses
 find_get_pages_contig

On Tue, Jun 09, 2020 at 05:47:53PM -0700, Matthew Wilcox wrote:
> On Tue, Jun 09, 2020 at 08:10:36PM -0400, Kent Overstreet wrote:
> > @@ -2275,83 +2287,93 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
> >  		struct iov_iter *iter, ssize_t written)
> >  {
> >  	struct file *filp = iocb->ki_filp;
> > +	struct file_ra_state *ra = &filp->f_ra;
> >  	struct address_space *mapping = filp->f_mapping;
> >  	struct inode *inode = mapping->host;
> > -	struct file_ra_state *ra = &filp->f_ra;
> >  	size_t orig_count = iov_iter_count(iter);
> > -	pgoff_t last_index;
> > -	int error = 0;
> > +	struct page *pages[64];
> 
> That's 512 bytes which seems like a lot of stack space.  Would 16 be
> enough to see a significant fraction of the benefit?

Ah right, we do call into fs code for readahead from here. I'll switch it to
kmalloc the page array if it's more than 16.