Re: [PATCH v4 3/3] fs: add support for SquashFS 4.0

[Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>]

On Fri, Feb 26, 2016 at 04:19:45PM +0100, yegorslists@xxxxxxxxxxxxxx wrote:
> From: Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx>
> 
> The driver was imported from Linux 4.4.
> 
> Current implementation supports only XZ decompressor.
> 
> Tested-by: Antony Pavlov <antonynpavlov@xxxxxxxxx>
> Signed-off-by: Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx>
> ---
> Changes:
> 	v4: - add promt to mount examples
> 	    - rework squashfs_read to enable non linear reading
> 	    - embed super_block into squashfs_priv
> 	    - check malloc' return value
> 
>         v3: - rework inode searching
>             - eliminate memleaks in directory search
> 
>         v2: - fix checkpatch errors
>             - remove spinlock/wait code from cache implementation
>             - fix directory listing
>             - add documentation entry about SquashFS
>             - fix compiling without XZ enabled
>             - fix some memory leaks
> 
> +
> +static int squashfs_read(struct device_d *_dev, FILE *f, void *buf,
> +		size_t insize)
> +{
> +	unsigned int size = insize;
> +	unsigned int pos = f->pos;
> +	unsigned int ofs;
> +	unsigned int now;
> +	unsigned int idx;
> +	unsigned int data_block_pos, data_block;
> +	struct squashfs_page *page = f->priv;
> +
> +	data_block = f->pos / (32 * PAGE_CACHE_SIZE);
> +	if (data_block != page->data_block) {
> +		page->data_block = data_block;
> +		page->idx = 0;
> +	}
> +
> +	data_block_pos = f->pos - page->data_block * 32 * PAGE_CACHE_SIZE;
> +	idx = data_block_pos / PAGE_CACHE_SIZE;
> +	page->real_page.index = page->data_block * 32;
> +
> +	while (size) {
> +		if (page->idx == 0)
> +			squashfs_readpage(NULL, &page->real_page);
> +
> +		/* Read till end of current buffer page */
> +		ofs = pos % PAGE_CACHE_SIZE;
> +		if (ofs) {
> +			now = min(size, PAGE_CACHE_SIZE - ofs);
> +			memcpy(buf, page->buf[idx] + ofs, now);
> +
> +			size -= now;
> +			pos += now;
> +			buf += now;
> +			idx++;
> +			if (idx > 31) {
> +				idx = 0;
> +				page->idx = 0;
> +				page->data_block++;
> +				page->real_page.index = page->data_block * 32;
> +				continue;
> +			}
> +		}
> +
> +		/* Do full buffer pages */
> +		while (size >= PAGE_CACHE_SIZE) {
> +			memcpy(buf, page->buf[idx], PAGE_CACHE_SIZE);
> +			size -= PAGE_CACHE_SIZE;
> +			pos += PAGE_CACHE_SIZE;
> +			buf += PAGE_CACHE_SIZE;
> +			idx++;
> +			if (idx > 31) {
> +				idx = 0;
> +				page->idx = 0;
> +				page->data_block++;
> +				page->real_page.index = page->data_block * 32;
> +				continue;

Here you increase the real_page.index without calling
squashfs_readpage() again.

The outer loop shouldn't be necessary. It is used in the if (ofs) above,
but this doesn't look entirely correct.

I think you are doing too much in this function. It should be easier if
you create a helper function that takes the current file position and
returns the buffer that belongs to this position. See below for a (completely
untested) template:

static int squashfs_read_buf(struct squashfs_page *page, int pos, void **buf)
{
	unsigned int data_block = pos / (32 * PAGE_CACHE_SIZE);
	unsigned int data_block_pos = pos % (32 * PAGE_CACHE_SIZE);
	unsigned int idx = data_block_pos / PAGE_CACHE_SIZE;

	if (data_block != page->data_block) {
		page->real_page.index = data_block * 32;
		squashfs_readpage(NULL, &page->real_page);
		page->data_block = data_block;
	}

	*buf = page->buf[idx];

	return 0;
}

static int squashfs_read(struct device_d *_dev, FILE *f, void *buf,
		size_t insize)
{
	unsigned int size = insize;
	unsigned int pos = f->pos;
	unsigned int ofs;
	unsigned int now;
	void *pagebuf;
	struct squashfs_page *page = f->priv;

	/* Read till end of current buffer page */
	ofs = pos % PAGE_CACHE_SIZE;
	if (ofs) {
		squashfs_read_buf(page, pos, &pagebuf);

		now = min(size, PAGE_CACHE_SIZE - ofs);
		memcpy(buf, pagebuf + ofs, now);

		size -= now;
		pos += now;
		buf += now;
	}

	/* Do full buffer pages */
	while (size >= PAGE_CACHE_SIZE) {
		squashfs_read_buf(page, pos, &pagebuf);

		memcpy(buf, pagebuf, PAGE_CACHE_SIZE);
		size -= PAGE_CACHE_SIZE;
		pos += PAGE_CACHE_SIZE;
		buf += PAGE_CACHE_SIZE;
	}

	/* And the rest */
	if (size) {
		squashfs_read_buf(page, pos, &pagebuf);
		memcpy(buf, pagebuf, size);
		size  = 0;
	}

	return insize;
}

Sascha

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