On Wed, Jul 21, 2021 at 11:22 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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. > Hi Matthew, I don't yet know if this can be useful in other scenarios. But for the above scenario (of eagerly calling readahead), I thought that this info can be used by a filesystem for throttling, which it doesn't get today. I was also thinking that there could potentially be other classifications, apart from sync vs async, for example the process IO priority. Today, I don't see the process IO priority used by block layer, and not in vfs or the individual filesystems. Do you think this is also another info that could/should trickle down to individual filesystems? CCing fsdevel also to get more inputs on this. > 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. -- Regards, Shyam
