Re: [PATCHSET v8 0/12] Uncached buffered IO

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 13 Jan 2025 08:34:18 -0700 Jens Axboe <axboe@kernel.dk> 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?