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