Messages in this thread | | | Date | Sat, 2 Jun 2007 18:35:53 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH] sendfile removal |
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On Sat, Jun 02 2007, Linus Torvalds wrote: > > > On Sat, 2 Jun 2007, Jens Axboe wrote: > > > > splice() WILL return EAGAIN, but in that case it should have triggered > > the read-ahead and thus started some IO. > > That's not enough. > > If the IO has already been started, splice needs to wait.
But splice doesn't know, page_cache_readahead() may not have started anything.
> > For the from-file case, see __generic_file_splice_read(). splice does: > > > > if (!PageUptodate(page)) { > > /* > > * If in nonblock mode then dont block on > > * waiting > > * for an in-flight io page > > */ > > if (flags & SPLICE_F_NONBLOCK) { > > if (TestSetPageLocked(page)) > > break; > > } else > > lock_page(page); > > Yeah, that's just wrong. > > Your suggested: > > > if ((flags & SPLICE_F_NONBLOCK) && spd.nr_pages) { > > if (TestSetPageLocked(page)) > > break; > > } else > > lock_page(page); > > > > should do that - always block for the first page and potentially return > > a partial results for the remaining pages that read-ahead kicked into > > gear. > > would work, but I suspect that for a server, returning EAGAIN once is > actually the best option - if it has a select() loop, and something else > is running, the "return EAGAIN once" actually makes tons of sense (it > would basically boil down to the kernel effectively saying "ok, try > anything else you might have pending in your queues first, if you get back > to me, I'll block then").
Well then the current code should work, _provided_ that we know read-ahead kicked off the IO. Well almost, still needs a bit of tweaking, with some knowledge of whether page_cache_readahead() actually called into read_pages() or not.
-- Jens Axboe
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