On Mon, 13 Jan 2025 08:34:18 -0700 Jens Axboe <axboe@xxxxxxxxx> wrote: > > > > ... > > > Of course, we're doing something here which userspace could itself do: > > drop the pagecache after reading it (with appropriate chunk sizing) and > > for writes, sync the written area then invalidate it. Possible > > added benefits from using separate threads for this. > > > > I suggest that diligence requires that we at least justify an in-kernel > > approach at this time, please. > > Conceptually yes. But you'd end up doing extra work to do it. Some of > that not so expensive, like system calls, and others more so, like LRU > manipulation. Outside of that, I do think it makes sense to expose as a > generic thing, rather than require applications needing to kick > writeback manually, reclaim manually, etc. > > > And there's a possible middle-ground implementation where the kernel > > itself kicks off threads to do the drop-behind just before the read or > > write syscall returns, which will probably be simpler. Can we please > > describe why this also isn't acceptable? > > That's more of an implementation detail. I didn't test anything like > that, though we surely could. If it's better, there's no reason why it > can't just be changed to do that. My gut tells me you want the task/CPU > that just did the page cache additions to do the pruning to, that should > be more efficient than having a kworker or similar do it. Well, gut might be wrong ;) There may be benefit in using different CPUs to perform the dropbehind, rather than making the read() caller do this synchronously. If I understand correctly, the write() dropbehind is performed at interrupt (write completion) time so that's already async. > > Also, it seems wrong for a read(RWF_DONTCACHE) to drop cache if it was > > already present. Because it was presumably present for a reason. Does > > this implementation already take care of this? To make an application > > which does read(/etc/passwd, RWF_DONTCACHE) less annoying? > > The implementation doesn't drop pages that were already present, only > pages that got created/added to the page cache for the operation. So > that part should already work as you expect. > > > Also, consuming a new page flag isn't a minor thing. It would be nice > > to see some justification around this, and some decription of how many > > we have left. > > For sure, though various discussions on this already occurred and Kirill > posted patches for unifying some of this already. It's not something I > wanted to tackle, as I think that should be left to people more familiar > with the page/folio flags and they (sometimes odd) interactions. Matthew & Kirill: are you OK with merging this as-is and then revisiting the page-flag consumption at a later time?
