On 2022/3/5 3:32, Mike Kravetz wrote: > On 3/4/22 00:26, HORIGUCHI NAOYA(堀口 直也) wrote: >> On Mon, Feb 28, 2022 at 10:02:42PM +0800, Miaohe Lin wrote: >>> There is a race window where we got the compound_head, the hugetlb page >>> could be freed to buddy, or even changed to another compound page just >>> before we try to get hwpoison page. If this happens, just bail out. >> >> I think that when some hugetlb page is about to change into other type/size >> of compound page, it has to go through buddy allocator because hugetlb pages >> are maintained in separate memory allocator and they never change into other >> normal state directly. memory_failure_hugetlb() takes refcount before >> lock_page(), so the hugetlb page seems not change between get_hwpoison_page() >> and lock_page(). So it this new check really necessary? > > A hugetlb page could change size without going through buddy via the new > demote functionality [1]. Only hugetlb pages on the hugetlb free list can > be demoted. > > We should not demote a page if poison is set. However, there is no check in > the demote code. IIUC, poison is set early in the memory error handling > process, even before taking ref on page. Demote code needs to be fixed so > that poisoned pages are not demoted. I can do that. > > With this change in place, then I think Naoya's statement that hugetlb pages > can not change state is correct and this patch is not necessary. > Sorry for my confusing commit words. What I mean to tell is indeed the below race: CPU 1 CPU 2 memory_failure_hugetlb struct page *head = compound_head(p); hugetlb page is freed to buddy, or even changed to another compound page as we haven't held the page refcnt now get_hwpoison_page -- page is not what we want now... Does this make sense for both of you? Many thanks for comment and reply! :) > Does that sound reasonable? > > [1] https://lore.kernel.org/linux-mm/20211007181918.136982-1-mike.kravetz@xxxxxxxxxx/ This is really a nice feature. Thanks. >
