Asynchronous read

[Da Zheng]

Hello,

I'm trying to understand the read operation in VFS, and get confused by the
asynchronous and synchronous operations.

At the beginning, do_sync_read() invokes aio_read, which is
generic_file_aio_read for ext4. generic_file_aio_read should be asynchronous
read. But what really confuses me is do_generic_file_read, which is called by
generic_file_aio_read. It seems to me do_generic_file_read implements
synchronous read as this is the only function I can find that copy data to the
user space by invoking the actor callback function. If do_generic_file_read is
synchronous, how can generic_file_aio_read be asynchronous?

In do_generic_file_read, if the data to be read isn't in the cache, normally
page_cache_sync_readahead should be called. As far as I understand, when
page_cache_sync_readahead returns, the pages will be ready in the cache, but the
corresponding data in the disk isn't necessarily copied to the pages yet
(because it eventually only invokes submit_bio to submit the IO requests to the
block layer), so PageUptodate of the requested page might still return false,
and then do_generic_file_read tries to invoke readpage to read the page again
instead of waiting. Since the disk is always very slow, doesn't it just waste
CPU time? Or do I miss something?

Best,
Da