On Mon, May 18, 2015 at 01:49:39PM +0200, Vlastimil Babka wrote:
> On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> >With new THP refcounting, we don't need tricks to stabilize huge page.
> >If we've got reference to tail page, it can't split under us.
> >
> >This patch effectively reverts a5b338f2b0b1.
> >
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> >---
> > kernel/futex.c | 61 ++++++++++++----------------------------------------------
> > 1 file changed, 12 insertions(+), 49 deletions(-)
> >
> >diff --git a/kernel/futex.c b/kernel/futex.c
> >index f4d8a85641ed..cf0192e60ef9 100644
> >--- a/kernel/futex.c
> >+++ b/kernel/futex.c
> >@@ -399,7 +399,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
> > {
> > unsigned long address = (unsigned long)uaddr;
> > struct mm_struct *mm = current->mm;
> >- struct page *page, *page_head;
> >+ struct page *page;
> > int err, ro = 0;
> >
> > /*
> >@@ -442,46 +442,9 @@ again:
> > else
> > err = 0;
> >
> >-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >- page_head = page;
> >- if (unlikely(PageTail(page))) {
> >- put_page(page);
> >- /* serialize against __split_huge_page_splitting() */
> >- local_irq_disable();
> >- if (likely(__get_user_pages_fast(address, 1, !ro, &page) == 1)) {
> >- page_head = compound_head(page);
> >- /*
> >- * page_head is valid pointer but we must pin
> >- * it before taking the PG_lock and/or
> >- * PG_compound_lock. The moment we re-enable
> >- * irqs __split_huge_page_splitting() can
> >- * return and the head page can be freed from
> >- * under us. We can't take the PG_lock and/or
> >- * PG_compound_lock on a page that could be
> >- * freed from under us.
> >- */
> >- if (page != page_head) {
> >- get_page(page_head);
> >- put_page(page);
> >- }
> >- local_irq_enable();
> >- } else {
> >- local_irq_enable();
> >- goto again;
> >- }
> >- }
> >-#else
> >- page_head = compound_head(page);
> >- if (page != page_head) {
> >- get_page(page_head);
> >- put_page(page);
> >- }
>
> Hmm, any idea why this was there? Without THP, it was already sure that
> get/put_page() on tail page operates on the head page's _count, no?
I guess it's just to deal with the same page from this point forward.
Pin/unpin one page, but lock other could look strange.
--
Kirill A. Shutemov
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