On Thu, Nov 29, 2018 at 9:51 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: > > > > On 2018-11-29 10:30 a.m., Dan Williams wrote: > > Oh! Yes, nice find. We need to wait for the percpu-ref to be dead and > > all outstanding references dropped before we can proceed to > > arch_remove_memory(), and I think this problem has been there since > > day one because the final exit was always after devm_memremap_pages() > > release which means arch_remove_memory() was always racing any final > > put_page(). I'll take a look, it seems the arch_remove_pages() call > > needs to be moved out-of-line to its own context and wait for the > > final exit of the percpu-ref. > > Ok, well I thought moving the wait_for_completion() into the kill() call > was a pretty good solution to this. True, it is... > Though, if we move the > arch_remove_pages() into a different context, it *may* help with the > problem below... Glad to see my over-engineered proposal in this case might be good for something... > > >> Though, now that I look at it, the current change in question will be > >> wrong if there are two devm_memremap_pages_release()s to call. Both need > >> to drop their references before we can wait_for_completion() ;(. I guess > >> I need multiple percpu_refs or more complex changes to > >> devm_memremap_pages_release(). > > > > Can you just have a normal device-level kref for this case? On final > > device-level kref_put then kill the percpu_ref? I guess the problem is > > devm semantics where p2pdma only gets one callback on a driver > > ->remove() event. I'm not sure how to support multiple references of > > the same pages without creating a non-devm version of > > devm_memremap_pages(). I'm not opposed to that, but afaiu I don't > > think p2pdma is compatible with devm as long as it supports N>1:1 > > mappings of the same range. > > Hmm, no I think you misunderstood what I said. I'm saying I need to have > exactly one percpu_ref per call to devm_memremap_pages() and this is > doable, just slightly annoying. Right now I have one percpu_ref for > multiple calls to devm_memremap_pages() which doesn't work with the > above fix because there will always be a wait_for_completion() before > the last references are dropped in this way: > > 1) First devm_memremap_pages_release() is called which drops it's > reference and waits_for_completion(). > > 2) The second devm_memremap_pages_release() needs to be called to drop > it's reference, but can't seeing the first is waiting, and therefore the > percpu_ref never goes to zero and the wait_for_completion() never returns. > Got it, let me see how bad moving arch_remove_memory() turns out, sounds like a decent approach to coordinate multiple users of a single ref.
