linux-kernel - Re: [PATCH v5 03/22] cachefiles: introduce on-demand read mode

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Date:   Mon, 21 Mar 2022 22:16:22 +0800
From:   JeffleXu <jefflexu@...ux.alibaba.com>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-cachefs@...hat.com, xiang@...nel.org, chao@...nel.org,
        linux-erofs@...ts.ozlabs.org, torvalds@...ux-foundation.org,
        gregkh@...uxfoundation.org, willy@...radead.org,
        linux-fsdevel@...r.kernel.org, joseph.qi@...ux.alibaba.com,
        bo.liu@...ux.alibaba.com, tao.peng@...ux.alibaba.com,
        gerry@...ux.alibaba.com, eguan@...ux.alibaba.com,
        linux-kernel@...r.kernel.org, luodaowen.backend@...edance.com
Subject: Re: [PATCH v5 03/22] cachefiles: introduce on-demand read mode

Hi,

Thanks for reviewing.


On 3/21/22 9:34 PM, David Howells wrote:
> Jeffle Xu <jefflexu@...ux.alibaba.com> wrote:
> 
>> Fscache/cachefiles used to serve as a local cache for remote fs. This
>> patch, along with the following patches, introduces a new on-demand read
>> mode for cachefiles, which can boost the scenario where on-demand read
>> semantics is needed, e.g. container image distribution.
>>
>> The essential difference between the original mode and on-demand read
>> mode is that, in the original mode, when cache miss, netfs itself will
>> fetch data from remote, and then write the fetched data into cache file.
>> While in on-demand read mode, a user daemon is responsible for fetching
>> data and then writing to the cache file.
>>
>> This patch only adds the command to enable on-demand read mode. An optional
>> parameter to "bind" command is added. On-demand mode will be turned on when
>> this optional argument matches "ondemand" exactly, i.e.  "bind
>> ondemand". Otherwise cachefiles will keep working in the original mode.
> 
> You're not really adding a command, per se.  Also, I would recommend
> starting the paragraph with a verb.  How about:
> 
> 	Make it possible to enable on-demand read mode by adding an
> 	optional parameter to the "bind" command.  On-demand mode will be
> 	turned on when this parameter is "ondemand", i.e. "bind ondemand".
> 	Otherwise cachefiles will work in the original mode.
> 
> Also, I'd add a note something like the following:
> 
> 	This is implemented as a variation on the bind command so that it
> 	can't be turned on accidentally in /etc/cachefilesd.conf when
> 	cachefilesd isn't expecting it.	

Alright, looks much better :)

> 
>> The following patches will implement the data plane of on-demand read
>> mode.
> 
> I would remove this line.  If ondemand mode is not fully implemented in
> cachefiles at this point, I would be tempted to move this to the end of the
> cachefiles subset of the patchset.  That said, I'm not sure it can be made
> to do anything much before that point.


Alright.

> 
>> +#ifdef CONFIG_CACHEFILES_ONDEMAND
>> +static inline void cachefiles_ondemand_open(struct cachefiles_cache *cache)
>> +{
>> +	xa_init_flags(&cache->reqs, XA_FLAGS_ALLOC);
>> +	rwlock_init(&cache->reqs_lock);
>> +}
> 
> Just merge that into the caller.
> 
>> +static inline void cachefiles_ondemand_release(struct cachefiles_cache *cache)
>> +{
>> +	xa_destroy(&cache->reqs);
>> +}
> 
> Ditto.
> 
>> +static inline
>> +bool cachefiles_ondemand_daemon_bind(struct cachefiles_cache *cache, char *args)
>> +{
>> +	if (!strcmp(args, "ondemand")) {
>> +		set_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags);
>> +		return true;
>> +	}
>> +
>> +	return false;
>> +}
>> ...
>> +	if (!cachefiles_ondemand_daemon_bind(cache, args) && *args) {
>> +		pr_err("'bind' command doesn't take an argument\n");
>> +		return -EINVAL;
>> +	}
>> +
> 
> I would merge these together, I think, and say something like "Ondemand
> mode not enabled in kernel" if CONFIG_CACHEFILES_ONDEMAND=n.
> 

The reason why I extract all these logic into small sized function is
that, the **callers** can call cachefiles_ondemand_daemon_bind()
directly without any clause like:

```
#ifdef CONFIG_CACHEFILES_ONDEMAND
	...
#else
	...
```



Another choice is like

```
if (IS_ENABLED(CONFIG_CACHEFILES_ONDEMAND))
	...
else
	...
```


-- 
Thanks,
Jeffle