On Wed, Jul 21, 2021 at 10:38:59PM +0530, Shyam Prasad N wrote: > In a scenario where a user/application issues a readahead/fadvise for > large data ranges in advance (informing the kernel that they intend to > read these data ranges soon). Depending on how much data ranges these > calls cover, it could keep the network quite busy for a network > filesystem (or the disk for a block filesystem). > > I see some value if filesystems have the ability to differentiate the > reads from regular buffered reads by users. In such cases, the > filesystem can choose to throttle the readahead reads, so that there's > a specified bandwidth that's still available for regular reads. > > I wanted to get your opinions about this. And whether this can be done > already in VFS ->readahead and ->readpage calls in the filesystems? This is something I have an interest in, but haven't had time to pursue. The readahead code gets this information because the page cache calls page_cache_sync_ra() if it needs this page right now, and calls page_cache_async_ra() if it thinks it will need the page in the future. ondemand_readahead() currently gets a true/false parameter (hit_readahead_marker), although my folio patches change it to pass in a folio or NULL. That is then *not* passed to the filesystem, but it could be information passed in the ractl. There's also some tidying-up to be done around faulting. Currently fault-around doesn't have a way to express "read me all the pages around page N". Instead it just assumes that pages N-R/2 to N+R/2 are the right ones to fetch when it should be left up to the filesystem or the readahead code to determine what window of pages to fetch. Another thing I have an interest in doing but not had opportunity to pursue is making ->readpage synchronous. The current MM code always calls ->readahead first and only calls ->readpage if ->readahead fails. That means that all the async ->readpage work is actually wrong; we want to return the best error possible from ->readpage, even if that means sleeping. Oh ... except for swap. For NFS only, it calls ->readpage, so it really wants ->readpage to be async so it can kick off multiple pages and then wait for the one it actually needs. That gets into a conversation about how much we really care about swap-over-NFS, whether swap should be using ->readpage or ->direct_IO, and whether swap should use the file readahead code or its own virtual address based readahead code. Most of those discussions are outside my area of expertise.
