Re: [PATCH v3 3/3] squashfs: implement readahead

[Phillip Lougher]

On 31/05/2022 21:47, Andrew Morton wrote:
> On Mon, 23 May 2022 14:59:13 +0800 Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> > Implement readahead callback for squashfs. It will read datablocks
> > which cover pages in readahead request. For a few cases it will
> > not mark page as uptodate, including:
> > - file end is 0.
> > - zero filled blocks.
> > - current batch of pages isn't in the same datablock or not enough in a
> >    datablock.
> > - decompressor error.
> > Otherwise pages will be marked as uptodate. The unhandled pages will be
> > updated by readpage later.
> >
> > ...
>
> The choice of types seems somewhat confused.
>
> > @@ -495,7 +496,95 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
> >   	return 0;
> >   }
> >   
> > +static void squashfs_readahead(struct readahead_control *ractl)
> > +{
> > +	struct inode *inode = ractl->mapping->host;
> > +	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
> > +	size_t mask = (1UL << msblk->block_log) - 1;
> > +	size_t shift = msblk->block_log - PAGE_SHIFT;
>
> block_log is unsigned short.  Why size_t?
>
> > +	loff_t start = readahead_pos(ractl) &~ mask;
> > +	size_t len = readahead_length(ractl) + readahead_pos(ractl) - start;
> > +	struct squashfs_page_actor *actor;
> > +	unsigned int nr_pages = 0;
>
> OK.
>
> > +	struct page **pages;
> > +	u64 block = 0;
> > +	int bsize, res, i, index, bytes, expected;
>
> `res' could be local to the inner loop.
>
> `i' is used in situations where an unsigned type would be more
> appropriate.  If it is made unsigned then `i' is no longer a suitable
> identifier.  Doesn't matter much.
>
> `index' is from page.index, which is pgoff_t.
>
> `bytes' could be local to the innermost loop.
>
> `expected' is inappropriately a signed type and could be local to the
> inner loop.
>
> > +	int file_end = i_size_read(inode) >> msblk->block_log;
> > +	unsigned int max_pages = 1UL << shift;
> > +	void *pageaddr;
> > +

pageaddr could be made local to the innermost scope.

Apart from that the patch and updated error handling looks
good.

Phillip

> > +	readahead_expand(ractl, start, (len | mask) + 1);
> > +
> > +	if (file_end == 0)
> > +		return;
> > +
> > +	pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
> > +	if (!pages)
> > +		return;
> > +
> > +	actor = squashfs_page_actor_init_special(pages, max_pages, 0);
> > +	if (!actor)
> > +		goto out;
> > +
> > +	for (;;) {
> > +		nr_pages = __readahead_batch(ractl, pages, max_pages);
> > +		if (!nr_pages)
> > +			break;
> > +
> > +		if (readahead_pos(ractl) >= i_size_read(inode) ||
> > +		    nr_pages < max_pages)
> > +			goto skip_pages;
> > +
> > +		index = pages[0]->index >> shift;
> > +		if ((pages[nr_pages - 1]->index >> shift) != index)
> > +			goto skip_pages;
> > +
> > +		expected = index == file_end ?
> > +			   (i_size_read(inode) & (msblk->block_size - 1)) :
> > +			    msblk->block_size;
> > +
> > +		bsize = read_blocklist(inode, index, &block);
> > +		if (bsize == 0)
> > +			goto skip_pages;
> > +
> > +		res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
> > +					 actor);
> > +
> > +		if (res == expected) {
> > +			/* Last page may have trailing bytes not filled */
> > +			bytes = res % PAGE_SIZE;
> > +			if (bytes) {
> > +				pageaddr = kmap_atomic(pages[nr_pages - 1]);
> > +				memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
> > +				kunmap_atomic(pageaddr);
> > +			}
> > +
> > +			for (i = 0; i < nr_pages; i++)
> > +				SetPageUptodate(pages[i]);
> > +		}
>
> res == -EIO is unhandled?
>
> > +		for (i = 0; i < nr_pages; i++) {
> > +			unlock_page(pages[i]);
> > +			put_page(pages[i]);
> > +		}
> > +	}
> > +
> > +	kfree(actor);
> > +	kfree(pages);
> > +	return;
> > +
> > +skip_pages:
> > +	for (i = 0; i < nr_pages; i++) {
> > +		unlock_page(pages[i]);
> > +		put_page(pages[i]);
> > +	}
> > +
> > +	kfree(actor);
> > +out:
> > +	kfree(pages);
> > +}
> >   
> >   const struct address_space_operations squashfs_aops = {
> > -	.read_folio = squashfs_read_folio
> > +	.read_folio = squashfs_read_folio,
> > +	.readahead = squashfs_readahead
> >   };