On Mon, May 20, 2024 at 04:44:53PM -0700, Sidhartha Kumar wrote: > On 5/20/24 4:30 PM, Matthew Wilcox wrote: > > On Mon, May 20, 2024 at 03:44:07PM -0700, Sidhartha Kumar wrote: > > > All users have been converted to use the folio version of these macros, > > > we can safely remove the page based interface. > > > > Yay! > > > > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > There is only one remaining user of page-based Test version of these macros. > > in mm/memory-hotplug.c: > > if (!PageHuge(page)) > continue; > head = compound_head(page); > /* > * This test is racy as we hold no reference or lock. The > * hugetlb page could have been free'ed and head is no longer > * a hugetlb page before the following check. In such unlikely > * cases false positives and negatives are possible. Calling > * code must deal with these scenarios. > */ > if (HPageMigratable(head)) > goto found; > skip = compound_nr(head) - (pfn - page_to_pfn(head)); > > > I've previously sent a patch to convert this to folios[1] but got feedback > that it was unsafe. But I'm not sure why replacing compound_head() with > page_folio() and using folio_test_hugetlb_migratable(folio) rather than > HPageMigratable(head) changes the existing behavior. With no reference or > lock, can't the head pointer also be moved and no longer be a part of page > like the comment states. So would the folio conversion just be maintaining > this level of existing un-safety that the calling code should handle > anyways? To be fair, that wasn't the last thing I said about that ... https://lore.kernel.org/lkml/Y89DK23hYiLtgGNk@xxxxxxxxxxxxxxxxxxxx/ and looked like David agreed that this was a case where false postive/negative was fine; we were just looking to be right most of the time.
