Re: [PATCHv5 14/28] futex, thp: remove special case for THP in get_futex_key

["Kirill A. Shutemov" <kirill@shutemov.name>]

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@linux.intel.com>
> >Tested-by: Sasha Levin <sasha.levin@oracle.com>
> >---
> >  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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