[PATCH v3 05/22] cachefiles: introduce new devnode for on-demand read mode

[JeffleXu]

Hi David,

FYI I've updated this patch on [1].

[1]
https://github.com/lostjeffle/linux/commit/589dd838dc539aee291d1032406653a8f6269e6f.

This new version mainly adds cachefiles_ondemand_flush_reqs(), which
drains the pending read requests when cachefilesd is going to exit.

On 2/9/22 2:00 PM, Jeffle Xu wrote:
> This patch introduces a new devnode 'cachefiles_ondemand' to support the
> newly introduced on-demand read mode.
> 
> The precondition for on-demand reading semantics is that, all blob files
> have been placed under corresponding directory with correct file size
> (sparse files) on the first beginning. When upper fs starts to access
> the blob file, it will "cache miss" (hit the hole) and then turn to user
> daemon for preparing the data.
> 
> The interaction between kernel and user daemon is described as below.
> 1. Once cache miss, .ondemand_read() callback of corresponding fscache
>    backend is called to prepare the data. As for cachefiles, it just
>    packages related metadata (file range to read, etc.) into a pending
>    read request, and then the process triggering cache miss will fall
>    asleep until the corresponding data gets fetched later.
> 2. User daemon needs to poll on the devnode ('cachefiles_ondemand'),
>    waiting for pending read request.
> 3. Once there's pending read request, user daemon will be notified and
>    shall read the devnode ('cachefiles_ondemand') to fetch one pending
>    read request to process.
> 4. For the fetched read request, user daemon need to somehow prepare the
>    data (e.g. download from remote through network) and then write the
>    fetched data into the backing file to fill the hole.
> 5. After that, user daemon need to notify cachefiles backend by writing a
>    'done' command to devnode ('cachefiles_ondemand'). It will also
>    awake the previous asleep process triggering cache miss.
> 6. By the time the process gets awaken, the data has been ready in the
>    backing file. Then process can re-initiate a read request from the
>    backing file.
> 
> Signed-off-by: Jeffle Xu <jefflexu at linux.alibaba.com>
> ---


-- 
Thanks,
Jeffle