Messages in this thread | | | Date | Thu, 30 Mar 2006 09:00:09 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH][RFC] splice support |
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On Thu, Mar 30 2006, Nick Piggin wrote: > Hi Jens, > > Looks nice! > > Jens Axboe wrote: > > >Hi, > > > >Since my initial posting back in December, I've had some private queries > >about the state of splice support. The state was pretty much that it was > >a little broken, if one attempted to do file | file splicing. The > >original patch migrated pages from one file to another in this case, > >which got vm ugly really quickly. And it wasn't always the right thing > >to do, since it would mean that splicing file1 to file2 would move > >file1's page cache to file2. Sometimes this is what you want, sometimes > >it is > > > > Page migration now generalised vmscan.c and introduced remove_mapping > function, which should help keep things clean.
Excellent.
> Moving a page onto and off the LRU is an interesting problem, though. > But possibly you could just leave it on the LRU and transfer the pagecache > reference over to the pipe. vmscan would find extra pages on the LRU at > times, but they would go away when pipe releases the page. > > Moving a page from a pipe to a filesystem might be harder, because you > don't know if it came from a filesystem (still on LRU) or not (in which > case you need to add it to LRU). If only you can keep track of this
Well that, to me, is _the_ hard problem to solve for this. But you sort-of do know, my plan is/was to add a ->steal() hook to the pipe buffers that would 'unhook' the page so it was in a clean state to be added to the LRU/page cache again. If stealing failed, just fall back to copying (or hard error, let the flags decide).
> information as the page gets passed around... hmm the PG_private will be > free to use because a filesystem must always drop its buffers before > remove_mapping can run. One would also need to take care of replacing > an existing page I guess. > > Hmm... I think it can work, falling back to copies if we get stuck > anywhere. > > Unless someone beats me to it, I'll try coding something up when I get > a bit more free time.
You are more than welcome, I hope to give it a little shot today and see how it goes.
-- Jens Axboe
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