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

[Mike Kravetz]

On 1/20/21 2:00 AM, Oscar Salvador wrote:
> 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@xxxxxxxxxx>
>>
>> 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.


Hi Oscar,

In Muchun's series of hugetlb bug fixes, Michal asked the same question.

https://lore.kernel.org/linux-mm/7e69a55c-d501-6b42-8225-a677f09fb829@xxxxxxxxxx/

The 'short answer' is that the this would allow a page to be migrated
after allocation but before the page fault code adds it to the page
cache or page tables.  This actually caused bugs in the past.
-- 
Mike Kravetz