On Mon, Jan 11, 2021 at 01:01:51PM -0800, Mike Kravetz wrote: > Use the new hugetlb page specific flag 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 interfaces were not really necessary > as in all case but one we KNOW the page is a hugetlb page. Therefore, > they are removed. In one call to HPageMigratable() is it possible for > the page to not be a hugetlb page due to a race. However, the code > making the call (scan_movable_pages) is inherently racy, and page state > will be validated later in the migration process. > > Note: Since HPageMigratable is used outside hugetlb.c, it can not be > static. Therefore, a new set of hugetlb page flag macros is added for > non-static flag functions. Two things about this one: I am not sure about the name of this one. It is true that page_huge_active() was only called by memory-hotplug and all it wanted to know was whether the page was in-use and so if it made sense to migrate it, so I see some value in the new PageMigratable flag. However, not all in-use hugetlb can be migrated, e.g: we might have constraints when it comes to migrate certain sizes of hugetlb, right? So setting HPageMigratable to all active hugetlb pages might be a bit misleading? HPageActive maybe? (Sorry, don't have a replacement) The other thing is that you are right that scan_movable_pages is racy, but page_huge_active() was checking if the page had the Head flag set before retrieving page[1]. Before the page_huge_active() in scan_movable_pages() we have the if (!PageHuge(page)) check, but could it be that between that check and the page_huge_active(), the page gets dissolved, and so we are checking a wrong page[1]? Am I making sense? -- Oscar Salvador SUSE L3
