linux-kernel - Re: [PATCH v3] iomap: support tail packing inline read

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Date:   Tue, 20 Jul 2021 00:11:49 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-erofs@...ts.ozlabs.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Andreas Gruenbacher <andreas.gruenbacher@...il.com>
Subject: Re: [PATCH v3] iomap: support tail packing inline read

On Mon, Jul 19, 2021 at 05:13:10PM +0200, Christoph Hellwig wrote:
> On Mon, Jul 19, 2021 at 04:02:30PM +0100, Matthew Wilcox wrote:
> > > +	if (iomap->type == IOMAP_INLINE) {
> > > +		iomap_read_inline_data(inode, page, iomap, pos);
> > > +		plen = PAGE_SIZE - poff;
> > > +		goto done;
> > > +	}
> > 
> > This is going to break Andreas' case that he just patched to work.
> > GFS2 needs for there to _not_ be an iop for inline data.  That's
> > why I said we need to sort out when to create an iop before moving
> > the IOMAP_INLINE case below the creation of the iop.
> > 
> > If we're not going to do that first, then I recommend leaving the
> > IOMAP_INLINE case where it is and changing it to ...
> > 
> > 	if (iomap->type == IOMAP_INLINE)
> > 		return iomap_read_inline_data(inode, page, iomap, pos);
> > 
> > ... and have iomap_read_inline_data() return the number of bytes that
> > it copied + zeroed (ie PAGE_SIZE - poff).
> 
> Returning the bytes is much cleaner anyway.  But we still need to deal
> with the sub-page uptodate status in one form or another.

There is another iomap_read_inline_data() caller as in:
+static int iomap_write_begin_inline(struct inode *inode, loff_t pos,
+		struct page *page, struct iomap *srcmap)
+{
+	/* needs more work for the tailpacking case, disable for now */
+	if (WARN_ON_ONCE(pos != 0))
+		return -EIO;
+	if (PageUptodate(page))
+		return 0;
+	iomap_read_inline_data(inode, page, srcmap, pos);
+	return 0;
+}

I'd like to avoid it as (void)iomap_read_inline_data(...). That's why it
left as void return type.

Anyway, let's check if it works correctly first. I think it's a high
priority stuff, and EROFS uncompressed cases can be entirely cleaned
up with iomap then.

Thanks,
Gao Xiang