On Wed, Mar 01, 2023 at 12:49:10PM +0800, Gao Xiang wrote:

quoted

The only problem is that the readahead code doesn't tell the filesystem
whether the request is sync or async.  This should be a simple matter
of adding a new 'bool async' to the readahead_control and then setting
REQ_RAHEAD based on that, rather than on whether the request came in
through readahead() or read_folio() (eg see mpage_readahead()).

Great!  In addition to that, just (somewhat) off topic, if we have a
"bool async" now, I think it will immediately have some users (such as
EROFS), since we'd like to do post-processing (such as decompression)
immediately in the same context with sync readahead (due to missing
pages) and leave it to another kworker for async readahead (I think
it's almost same for decryption and verification).

So "bool async" is quite useful on my side if it could be possible
passed to fs side.  I'd like to raise my hands to have it.

That's a really interesting use-case; thanks for bringing it up.

Ideally, we'd have the waiting task do the
decompression/decryption/verification for proper accounting of CPU.
Unfortunately, if the folio isn't uptodate, the task doesn't even hold
a reference to the folio while it waits, so there's no way to wake the
task and let it know that it has work to do.  At least not at the moment
... let me think about that a bit (and if you see a way to do it, feel
free to propose it)