Messages in this thread | | | Date | Tue, 10 May 2022 21:32:05 -0700 | Subject | Re: [PATCH v4] mm: fix is_pinnable_page against on cma page | From | John Hubbard <> |
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On 5/10/22 17:09, Minchan Kim wrote: > On Tue, May 10, 2022 at 04:58:13PM -0700, John Hubbard wrote: >> On 5/10/22 4:31 PM, Minchan Kim wrote: >>>>> + int __mt = get_pageblock_migratetype(page); >>>>> + int mt = __READ_ONCE(__mt); >>>> >>>> Although I saw the email discussion about this in v2, that discussion >>>> didn't go far enough. It started with "don't use volatile", and went >>>> on to "try __READ_ONCE() instead", but it should have continued on >>>> to "you don't need this at all". >>> >>> That's really what I want to hear from experts so wanted to learn >>> "Why". How could we prevent refetching of the mt if we don't use >>> __READ_ONCE or volatile there? >>> >>>> >>>> Because you don't. There is nothing you are racing with, and adding >>>> __READ_ONCE() in order to avoid a completely not-going-to-happen >>>> compiler re-invocation of a significant code block is just very wrong. >>>> >>>> So let's just let it go entirely. :) >>> >>> Yeah, once it's clear for everyone, I am happy to remove the >>> unnecessary lines. >>> >>>> >>>>> + >>>>> + if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE) >>>> >> >> With or without __READ_ONCE() or volatile or anything else, >> this code will do what you want. Which is: loosely check >> for either of the above. >> >> What functional problem do you think you are preventing >> with __READ_ONCE()? Because I don't see one. > > I discussed the issue at v1 so please take a look. > > https://lore.kernel.org/all/YnFvmc+eMoXvLCWf@google.com/
I read that, but there was never any real justification there for needing to prevent a re-read of mt, just a preference: "I'd like to keep use the local variable mt's value in folloing conditions checks instead of refetching the value from get_pageblock_migratetype."
But I don't believe that there is any combination of values of mt that will cause a problem here.
I also think that once we pull in experts, they will tell us that the compiler is not going to re-run a non-trivial function to re-fetch a value, but I'm not one of those experts, so that's still arguable. But imagine what the kernel code would look like if every time we call a large function, we have to consider if it actually gets called some arbitrary number of times, due to (anti-) optimizations by the compiler. This seems like something that is not really happening.
thanks, -- John Hubbard NVIDIA
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