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

Oscar Salvador
2021-01-21T08:30:12Z

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>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> -/*
> - * Test to determine whether the hugepage is "active/in-use" (i.e. being linked
> - * to hstate->hugepage_activelist.)
> - *
> - * This function can be called for tail pages, but never returns true for them.
> - */
> -bool page_huge_active(struct page *page)
> -{
> - return PageHeadHuge(page) && PagePrivate(&page[1]);

This made me think once again.
I wonder if we could ever see a scenario where page[0] is a rightful page while
page[1] is poisoned/unitialized (poison_pages()).
A lot of things would have to happen between the two checks, so I do not see it
possible and as you mentioned earlier, the race is already there.

Just wanted to speak up my mind.

--
Oscar Salvador
SUSE L3