On Thu, May 05, 2016 at 05:24:06PM +0200, Andrea Arcangeli wrote:

On Thu, May 05, 2016 at 06:11:10PM +0300, Kirill A. Shutemov wrote:

quoted

Hm. How total_mapcount equal to NULL wouldn't lead to NULL-pointer
dereference inside page_trans_huge_mapcount()?

Sorry for the confusion, this was still work in progress and then I've
seen the email from Alex and I sent the last version I had committed
right away. An earlier version of course had the proper checks for
NULL but they got wiped as I transitioned from one model to another
and back.

quoted

quoted

+				page_move_anon_rmap(old_page, vma, address);

compound_head() is missing, I believe.

Oh yes, fixed that too.

			if (total_mapcount == 1) {
				/*
				 * The page is all ours. Move it to
				 * our anon_vma so the rmap code will
				 * not search our parent or siblings.
				 * Protected against the rmap code by
				 * the page lock.
				 */
				page_move_anon_rmap(compound_head(old_page),
						    vma, address);
			}


If there's no other issue I can git send-email.

I don't see any.

Then we should look into calling page_move_anon_rmap from THP COWs
too, hugetlbfs calls it too. I think we probably need to make
page_move_anon_rmap smarter and optionally let it take the lock for us
after reading page->mapping first to be sure it's really moving it.

The question is then if trylock or lock_page should be used, my
preference would be just trylock.

trylock is probably fine. It's not big deal if we wouldn't move the page
to new anon_vma, just nice-to-have.

-- 
 Kirill A. Shutemov

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