Re: [RFC PATCH 7/9] jfs: Read quota through the page cache

[Matthew Wilcox <willy@infradead.org>]

On Thu, May 26, 2022 at 10:43:51PM -0700, Christoph Hellwig wrote:
> >  static ssize_t jfs_quota_read(struct super_block *sb, int type, char *data,
> > +			      size_t len, loff_t pos)
> >  {
> >  	struct inode *inode = sb_dqopt(sb)->files[type];
> > +	struct address_space *mapping = inode->i_mapping;
> >  	size_t toread;
> > +	pgoff_t index;
> >  	loff_t i_size = i_size_read(inode);
> >  
> > +	if (pos > i_size)
> >  		return 0;
> > +	if (pos + len > i_size)
> > +		len = i_size - pos;
> >  	toread = len;
> > +	index = pos / PAGE_SIZE;
> > +
> >  	while (toread > 0) {
> > +		struct folio *folio = read_mapping_folio(mapping, index, NULL);
> > +		size_t tocopy = PAGE_SIZE - offset_in_page(pos);
> > +		void *src;
> > +
> > +		if (IS_ERR(folio))
> > +			return PTR_ERR(folio);
> > +
> > +		src = kmap_local_folio(folio, offset_in_folio(folio, pos));
> > +		memcpy(data, src, tocopy);
> > +		kunmap_local(src);
> 
> It would be great to have a memcpy_from_folio like the existing
> memcpy_from_page for this.

Yes, I agree.  It could copy more than a single page like
zero_user_segments() does.

> > +		folio_put(folio);
> >  
> >  		toread -= tocopy;
> >  		data += tocopy;
> > +		pos += tocopy;
> > +		index++;
> >  	}
> >  	return len;
> 
> And this whole helper is generic now.  It might be worth to move it
> into fs/quota/dquot.c as generic_quota_read.

I was thinking it was filemap_read_kernel(inode, pos, dst, len)
but perhaps both of these things ...