linux-ext4 - Re: [PATCH V2 2/7] Cleancache (was Transcendent Memory): core files

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Date:	Thu, 10 Jun 2010 07:11:52 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	chris.mason@...cle.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, adilger@....com, tytso@....edu,
	mfasheh@...e.com, joel.becker@...cle.com, matthew@....cx,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	ocfs2-devel@....oracle.com, linux-mm@...ck.org, jeremy@...p.org,
	JBeulich@...ell.com, kurt.hackel@...cle.com, npiggin@...e.de,
	dave.mccracken@...cle.com, riel@...hat.com, avi@...hat.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH V2 2/7] Cleancache (was Transcendent Memory): core files

Hi,

On 05/28/2010 11:05 PM, Dan Magenheimer wrote:
> [PATCH V2 2/7] Cleancache (was Transcendent Memory): core files

I just finished a rough (but working) implementation of in-kernel
page cache compression backend (called zcache). During this work,
I found some issues with cleancache, mostly related to (lack of)
comments/documentation:


> +
> +static inline int cleancache_init_fs(size_t pagesize)
> +

 - It is not very obvious that this function is called when
an instance of cleancache supported filesystem is *mounted*.
Initially, I thought this is called which any such filesystem
module is loaded.

 - It seems that returning pool_id of 0 is considered as error
condition (as it appears from deactivate_locked_super() changes). 
This seems weird; I think only negative pool_id should considered
as error. Anyway, please add function comments for these.

> +int __cleancache_get_page(struct page *page)
> +{
> +	int ret = 0;
> +	int pool_id = page->mapping->host->i_sb->cleancache_poolid;
> +
> +	if (pool_id >= 0) {
> +		ret = (*cleancache_ops->get_page)(pool_id,
> +						  page->mapping->host->i_ino,
> +						  page->index,
> +						  page);
> +		if (ret == CLEANCACHE_GET_PAGE_SUCCESS)
> +			succ_gets++;
> +		else
> +			failed_gets++;
> +	}
> +	return ret;
> +}

It seems "non-standard" to use '1' as success code. You could simply use
0 for success and negative error code as failure. Then you can also get
rid of CLEANCACHE_GET_PAGE_SUCCESS.

> +
> +int __cleancache_put_page(struct page *page)

What return values stands for successful put? 1? Anyway, following the
same, 0 for success, negative codes for errors, seems to be better.

> +
> +int __cleancache_flush_page(struct address_space *mapping, struct page *page)

> +int __cleancache_flush_inode(struct address_space *mapping)

Return values for all the flush functions is ignored everywhere, so
why not make them return void instead?

> +static inline void cleancache_flush_fs(int pool_id)

Like init_fs, please document that it is called when a cleancache
aware filesystem is unmounted (or in other cases too?).



Page cache compression was a long-pending project. I'm glad its
coming into shape with the help of cleancache :)

Thanks,
Nitin

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