Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[Jason Gunthorpe]

On Wed, Sep 16, 2020 at 02:46:19PM -0400, Peter Xu wrote:

> My understanding is this may only work for the case when the fork()ed child
> quitted before we reach here (so we still have mapcount==1 for the
> page).  

Yes

> What if not?  Then mapcount will be greater than 1, and cow will
> still trigger.  Is that what we want?

That doesn't work today anyhow, so it is fine continuing to be broken.

> Another problem is that, aiui, one of the major change previous patch proposed
> is to avoid using lock_page() so that we never block in this path.

I saw you mention this before, but it looks like the change was to
lift some of the atomc_reads out of the lock and avoid the lock if
they indicate failure, checking also for page_maybe_dma_pinned()
outside the lock just means the rare case of FOLL_PIN we will take the
lock again.

> Maybe even more complicated, because "correctness" should be even harder
> than "best effort reuse" since it can cause data corruption if we didn't do it
> right...

The only correct way is for the application to avoid write protect on
FOLL_PIN pages. The purpose here is to allow applications that hadn't
hit "bad luck" and failed to keep working.

Another thought is to insert a warning print here as well that the
program is working improperly? At least it would give a transition
period to evaluate the extent of the problem.

We are thinking it is going to be a notable regression.

I botched the last version of the patch, here is something a bit
better.

Does it seem like it could be OK? I know very little about this part
of the kernel

Thanks,
Jason

diff --git a/mm/memory.c b/mm/memory.c
index 469af373ae76e1..332de777854f8b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2889,6 +2889,24 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
 	return ret;
 }
 
+static bool cow_needed(struct vm_fault *vmf)
+{
+	int total_map_swapcount;
+
+	if (!reuse_swap_page(vmf->page, &total_map_swapcount))
+		return true;
+
+	if (total_map_swapcount == 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(vmf->page, vmf->vma);
+	}
+	return false;
+}
+
 /*
  * This routine handles present pages, when users try to write
  * to a shared page. It is done by copying the page to a new address
@@ -2942,13 +2960,27 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		struct page *page = vmf->page;
 
 		/* PageKsm() doesn't necessarily raise the page refcount */
-		if (PageKsm(page) || page_count(page) != 1)
+		if (PageKsm(page))
 			goto copy;
+		if (page_count(page) != 1) {
+			/*
+			 * If the page is DMA pinned we can't rely on the
+			 * above to know if there are other CPU references as
+			 * page_count() will be elevated by the
+			 * pin. Needlessly copying the page will cause the DMA
+			 * pin to break, try harder to avoid that.
+			 */
+			if (!page_maybe_dma_pinned(page))
+				goto copy;
+		}
+
 		if (!trylock_page(page))
 			goto copy;
 		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
-			unlock_page(page);
-			goto copy;
+			if (cow_needed(vmf)) {
+				unlock_page(page);
+				goto copy;
+			}
 		}
 		/*
 		 * Ok, we've got the only map reference, and the only