linux-kernel - Re: [PATCH v6] iomap: support tail packing inline read

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Message-ID: <YPrtJLLsjvvQm1sD@B-P7TQMD6M-0146.local>
Date:   Sat, 24 Jul 2021 00:24:04 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, 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 v6] iomap: support tail packing inline read

On Fri, Jul 23, 2021 at 04:56:35PM +0100, Matthew Wilcox wrote:
> On Fri, Jul 23, 2021 at 11:23:38PM +0800, Gao Xiang wrote:
> > Hi Matthew,
> > 
> > On Fri, Jul 23, 2021 at 04:05:29PM +0100, Matthew Wilcox wrote:
> > > On Thu, Jul 22, 2021 at 07:39:47AM +0200, Christoph Hellwig wrote:
> > > > @@ -675,7 +676,7 @@ static size_t iomap_write_end_inline(struct inode *inode, struct page *page,
> > > >  
> > > >  	flush_dcache_page(page);
> > > >  	addr = kmap_atomic(page);
> > > > -	memcpy(iomap->inline_data + pos, addr + pos, copied);
> > > > +	memcpy(iomap_inline_buf(iomap, pos), addr + pos, copied);
> > > 
> > > This is wrong; pos can be > PAGE_SIZE, so this needs to be
> > > addr + offset_in_page(pos).
> > 
> > Yeah, thanks for pointing out. It seems so, since EROFS cannot test
> > such write path, previously it was disabled explicitly. I could
> > update it in the next version as above.
> 
> We're also missing a call to __set_page_dirty_nobuffers().  This
> matters to nobody right now -- erofs is read-only and gfs2 only
> supports inline data in the inode.  I presume what is happening
> for gfs2 is that at inode writeback time, it copies the ~60 bytes
> from the page cache into the inode and then schedules the inode
> for writeback.
> 
> But logically, we should mark the page as dirty.  It'll be marked
> as dirty by ->mkwrite, should the page be mmaped, so gfs2 must
> already cope with a dirty page for inline data.

I'd suggest we still disable tail-packing inline for buffered write
path until some real user for testing. I can see some (maybe) page
writeback, inode writeback and inline converting cases which is
somewhat complicated than just update like this.

I suggest it could be implemented with some real users, at least it can
provide the real write pattern and paths for testing. I will send the
next version like my previous version to disable it until some real fs
user cares and works out a real pattern.

Thanks,
Gao Xiang