Re: [PATCH v2 2/5] hugetlb: convert page_huge_active() HPageMigratable flag

Oscar Salvador
2021-01-20T10:30:31Z

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@oracle.com>
>
> 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.

--
Oscar Salvador
SUSE L3