Messages in this thread | | | Date | Wed, 11 May 2022 15:25:49 -0700 | Subject | Re: [PATCH v4] mm: fix is_pinnable_page against on cma page | From | John Hubbard <> |
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On 5/11/22 2:46 PM, Minchan Kim wrote: >> 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. > > Maybe, I might be paranoid since I have heard too subtle things > about how compiler could changes high level language code so wanted > be careful especially when we do lockless-stuff. > > Who cares when we change the large(?) function to small(?) function > later on? I'd like to hear from experts to decide it. >
Yes. But one thing that is still unanswered, that I think you can answer, is: even if the compiler *did* re-read the mt variable, what problems could that cause? I claim "no problems", because there is no combination of 0, _CMA, _ISOLATE, _CMA|ISOLATE that will cause problems here.
Any if that's true, then we can leave the experts alone, because the answer is there without knowing what happens exactly to mt.
thanks,
-- John Hubbard NVIDIA
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