On Wed, 12 Aug 2015 17:35:09 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote: > > > IIUC, the only potentially problematic callsites left are physical memory > > > scanners. This code requires audit. I'll do that. > > > > Please. > > I haven't finished the exercise yet. But here's an issue I believe present > in current *Linus* tree: > > >From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001 > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> > Date: Wed, 12 Aug 2015 17:09:16 +0300 > Subject: [PATCH] mm: fix potential race in isolate_migratepages_block() > > Hugh has pointed that compound_head() call can be unsafe in some context. > There's one example: > > CPU0 CPU1 > > isolate_migratepages_block() > page_count() > compound_head() > !!PageTail() == true > put_page() > tail->first_page = NULL > head = tail->first_page > alloc_pages(__GFP_COMP) > prep_compound_page() > tail->first_page = head > __SetPageTail(p); > !!PageTail() == true > <head == NULL dereferencing> > > The race is pure theoretical. I don't it's possible to trigger it in > practice. But who knows. > > This can be fixed by avoiding compound_head() in unsafe context. This is nuts :( page_count() should Just Work without us having to worry about bizarre races against splitting. Sigh. > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -787,7 +787,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > * admittedly racy check. > */ > if (!page_mapping(page) && > - page_count(page) > page_mapcount(page)) > + atomic_read(&page->_count) > page_mapcount(page)) > continue; If we're going to do this sort of thing, can we please do it in a more transparent manner? Let's not sprinkle unexplained and incomprehensible direct accesses to ->_count all over the place. Create a formal function to do this, with an appropriate name and with documentation which fully explains what's going on. Then use that here, and in has_unmovable_pages() (at least). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>