On 5/11/22 15:37, Minchan Kim wrote:
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.What scenario I am concerning with __READ_ONCE so compiler inlining get_pageblock_migratetype two times are CPU 0 CPU 1 alloc_contig_range is_pinnable_page start_isolate_page_range set_pageblock_migratetype(MIGRATE_ISOLATE) if (get_pageeblock_migratetype(page) == MIGRATE_CMA) so it's false undo: set_pageblock_migratetype(MIGRATE_CMA)if (get_pageeblock_migratetype(page) == MIGRATE_ISOLATE)so it's false In the end, CMA memory would be pinned by CPU 0 process so CMA allocation keep failed until the process release the refcount.
OK, so the code checks the wrong item each time. But the code really only needs to know "is either _CMA or _ISOLATE set?". And so you can just sidestep the entire question by writing it like this: int mt = get_pageblock_migratetype(page); if (mt & (MIGRATE_ISOLATE | MIGRATE_CMA)) return false; ...yes? thanks, -- John Hubbard NVIDIA
