linux-kernel - Re: [PATCH v3] squashfs: cache partial compressed blocks

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Date:   Wed, 17 May 2023 09:11:05 +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>, hch@....de,
        linux-kernel@...r.kernel.org, squashfs-devel@...ts.sourceforge.net,
        kernel@...s.com
Subject: Re: [PATCH v3] squashfs: cache partial compressed blocks

This looks generally good, but I have two more nitpicks (sorry for
beeing annoying):

> +	struct inode *cache_inode = msblk->cache_inode;
> +	struct address_space *cache_mapping = cache_inode ? cache_inode->i_mapping : NULL;
>
> +		struct page *page = NULL;
> +
> +		if (cache_mapping)
> +			page = find_get_page(cache_mapping,
> +					     read_start + i * PAGE_SIZE);

Given we only ever use the cache inode mapping, I'd suggest to store
that in the super_block and the just use mapping->host for the iput
on unmount.

> +		if (cache_mapping) {
> +			/*
> +			 * Use the __ version to avoid merging since we need
> +			 * each page to be separate when we check for and avoid
> +			 * cached pages.
> +			 */
> +			__bio_add_page(bio, page, len, offset);
> +		} else if (!bio_add_page(bio, page, len, offset)) {
>  			error = -EIO;
>  			goto out_free_bio;

And given that random alloc_page pages are basically never mergable
we should be able to just use __bio_add_page here unconditionally.