Re: [LSF/MM/BPF TOPIC] Cloud storage optimizations

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 01, 2023 at 01:09:34PM +0800, Gao Xiang wrote:
> On 2023/3/1 13:01, Matthew Wilcox wrote:
> > On Wed, Mar 01, 2023 at 12:49:10PM +0800, Gao Xiang wrote:
> > > > 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)
> 
> Honestly, I'd like to take the folio lock until all post-processing is
> done and make it uptodate and unlock so that only we need is to pass
> locked-folios requests to kworkers for async way or sync handling in
> the original context.
> 
> If we unlocked these folios in advance without uptodate, which means
> we have to lock it again (which could have more lock contention) and
> need to have a way to trace I/Oed but not post-processed stuff in
> addition to no I/Oed stuff.

Right, look at how it's handled right now ...

sys_read() ends up in filemap_get_pages() which (assuming no folio in
cache) calls page_cache_sync_readahead().  That creates locked, !uptodate
folios and asks the filesystem to fill them.  Unless that completes
incredibly quickly, filemap_get_pages() ends up in filemap_update_page()
which calls folio_put_wait_locked().

If the filesystem BIO completion routine could identify if there was
a task waiting and select one of them, it could wake up the waiter and
pass it a description of what work it needed to do (with the folio still
locked), rather than do the postprocessing itself and unlock the folio.

But that all seems _very_ hard to do with 100% reliability.  Note the
comment in folio_wait_bit_common() which points out that the waiters
bit may be set even when there are no waiters.  The wake_up code
doesn't seem to support this kind of thing (all waiters are
non-exclusive, but only wake up one of them).