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@xxxxxxxxxx/
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
