linux-kernel - Re: [PATCH v2] squashfs: cache partial compressed blocks

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Message-ID: <20230515100352.GA24402@lst.de>
Date:   Mon, 15 May 2023 12:03:52 +0200
From:   Christoph Hellwig <hch@....de>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     Phillip Lougher <phillip@...ashfs.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Philippe Liard <pliard@...gle.com>, hch@....de,
        linux-kernel@...r.kernel.org, squashfs-devel@...ts.sourceforge.net,
        kernel@...s.com
Subject: Re: [PATCH v2] squashfs: cache partial compressed blocks

On Fri, May 12, 2023 at 03:18:05PM +0200, Vincent Whitchurch wrote:
> +static int squashfs_bio_read_cached(struct bio *fullbio, struct address_space *cache_mapping,
> +				    u64 index, int length, u64 read_start, u64 read_end,
> +				    int page_count)

Please avoid the unreadable formatting and do something like:

static int squashfs_bio_read_cached(struct bio *fullbio,
		struct address_space *cache_mapping, u64 index, int length,
		u64 read_start, u64 read_end, int page_count)


> +		if (!bio || idx != prev_io_idx + 1) {
> +			unsigned int remaining_pages;
> +			unsigned int this_nr_pages;
> +
> +submit_and_retry:
> +			remaining_pages = page_count - idx;
> +			this_nr_pages = min(remaining_pages, BIO_MAX_VECS);
> +			bio = blk_next_bio(bio, bdev, this_nr_pages, REQ_OP_READ,
> +					   GFP_NOIO);
> +			bio->bi_iter.bi_sector = fullbio->bi_iter.bi_sector +
> +						 idx * (PAGE_SIZE / SECTOR_SIZE);
> +		}
> +
> +		retlen = bio_add_page(bio, bv->bv_page, bv->bv_len, bv->bv_offset);
> +		if (retlen != bv->bv_len)
> +			goto submit_and_retry;

Adding data payload to a bio while passing full bio_vec is usually
a sign that instead you should do a (partial) clone of the bio instead.
I think this is such a case.

>  static int squashfs_bio_read(struct super_block *sb, u64 index, int length,
>  			     struct bio **biop, int *block_offset)
>  {
>  	struct squashfs_sb_info *msblk = sb->s_fs_info;
> +	struct inode *cache_inode = msblk->cache_inode;
> +	struct address_space *cache_mapping = cache_inode ? cache_inode->i_mapping : NULL;

Unless I'm badly misreading the squashfs_fill_super changes,
cache_inode can't ever be NULL here or anywhere else in the I/O
code.