On 1/20/21 2:00 AM, Oscar Salvador wrote: > On Wed, Jan 20, 2021 at 10:59:05AM +0100, Oscar Salvador wrote: >> On Tue, Jan 19, 2021 at 05:30:46PM -0800, Mike Kravetz wrote: >>> Use the new hugetlb page specific flag HPageMigratable to replace the >>> page_huge_active interfaces. By it's name, page_huge_active implied >>> that a huge page was on the active list. However, that is not really >>> what code checking the flag wanted to know. It really wanted to determine >>> if the huge page could be migrated. This happens when the page is actually >>> added the page cache and/or task page table. This is the reasoning behind >>> the name change. >>> >>> The VM_BUG_ON_PAGE() calls in the *_huge_active() interfaces are not >>> really necessary as we KNOW the page is a hugetlb page. Therefore, they >>> are removed. >>> >>> The routine page_huge_active checked for PageHeadHuge before testing the >>> active bit. This is unnecessary in the case where we hold a reference or >>> lock and know it is a hugetlb head page. page_huge_active is also called >>> without holding a reference or lock (scan_movable_pages), and can race with >>> code freeing the page. The extra check in page_huge_active shortened the >>> race window, but did not prevent the race. Offline code calling >>> scan_movable_pages already deals with these races, so removing the check >>> is acceptable. Add comment to racy code. >>> >>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> >> >> Hi Mike, >> >> This comment addresses both this patch and the next one. >> >> Instead of putting the SetHPageMigratable flag spread over the >> allocation paths, would it make more sense to place it in >> alloc_huge_page before returning the page? >> Then we could opencode SetHPageMigratableIfSupported right there. > > and in putback_active_hugepage. Hi Oscar, In Muchun's series of hugetlb bug fixes, Michal asked the same question. https://lore.kernel.org/linux-mm/7e69a55c-d501-6b42-8225-a677f09fb829@xxxxxxxxxx/ The 'short answer' is that the this would allow a page to be migrated after allocation but before the page fault code adds it to the page cache or page tables. This actually caused bugs in the past. -- Mike Kravetz
