linux-kernel - Re: [PATCH 1/2] iomap: support tail packing inline read

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Date:   Sat, 17 Jul 2021 23:15:58 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-erofs@...ts.ozlabs.org,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Chao Yu <chao@...nel.org>,
        Liu Bo <bo.liu@...ux.alibaba.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Liu Jiang <gerry@...ux.alibaba.com>
Subject: Re: [PATCH 1/2] iomap: support tail packing inline read

Hi Matthew,

On Sat, Jul 17, 2021 at 04:01:38PM +0100, Matthew Wilcox wrote:
> On Sat, Jul 17, 2021 at 09:38:18PM +0800, Gao Xiang wrote:
> > Sorry about some late. I've revised a version based on Christoph's
> > version and Matthew's thought above. I've preliminary checked with
> > EROFS, if it does make sense, please kindly help check on the gfs2
> > side as well..
> 
> I don't understand how this bit works:

This part inherited from the Christoph version without change.
The following thoughts are just my own understanding...

> 
> >  	struct page *page = ctx->cur_page;
> > -	struct iomap_page *iop;
> > +	struct iomap_page *iop = NULL;
> >  	bool same_page = false, is_contig = false;
> >  	loff_t orig_pos = pos;
> >  	unsigned poff, plen;
> >  	sector_t sector;
> >  
> > -	if (iomap->type == IOMAP_INLINE) {
> > -		WARN_ON_ONCE(pos);
> > -		iomap_read_inline_data(inode, page, iomap);
> > -		return PAGE_SIZE;
> > -	}
> > +	if (iomap->type == IOMAP_INLINE && !pos)
> > +		WARN_ON_ONCE(to_iomap_page(page) != NULL);
> > +	else
> > +		iop = iomap_page_create(inode, page);
> 
> Imagine you have a file with bytes 0-2047 in an extent which is !INLINE
> and bytes 2048-2051 in the INLINE extent.  When you read the page, first
> you create an iop for the !INLINE extent.  Then this function is called

Yes, it first created an iop for the !INLINE extent.

> again for the INLINE extent and you'll hit the WARN_ON_ONCE.  No?

If it is called again with another INLINE extent, pos will be non-0?
so (!pos) == false. Am I missing something?

Thanks,
Gao Xiang

>