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/
