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).
