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

[Jason Gunthorpe]

On Tue, Sep 15, 2020 at 06:50:46PM -0700, John Hubbard wrote:
> > 
> > It seems very strange that a physical page exclusively owned by a
> > process can become copied if pin_user_pages() is active and the
> > process did fork() at some point.
> > 
> > Could the new pin_user_pages() logic help here? eg the
> > GUP_PIN_COUNTING_BIAS stuff?
> > 
> > Could the COW code consider a refcount of GUP_PIN_COUNTING_BIAS + 1 as
> > being owned by the current mm and not needing COW? The DMA pin would
> > be 'invisible' for COW purposes?
> 
> 
> Please do be careful to use the API, rather than the implementation. The
> FOLL_PIN refcounting system results in being able to get a "maybe
> DMA-pinned", or a "definitely not DMA-pinned", via this API call:

So, what I'm thinking is something like (untested):

diff --git a/mm/memory.c b/mm/memory.c
index 469af373ae76e1..77f63183667e52 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2889,6 +2889,26 @@ 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)) {
+		unlock_page(vmf->page);
+		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
@@ -2947,8 +2967,21 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		if (!trylock_page(page))
 			goto copy;
 		if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
+			bool cow = true;
+
+			/*
+			 * 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))
+				cow = cow_needed(vmf);
+
 			unlock_page(page);
-			goto copy;
+			if (cow)
+				goto copy;
 		}
 		/*
 		 * Ok, we've got the only map reference, and the only

What do you think Peter? Is this remotely close?

Seems like it preserves the fast path in most cases, the page_count &
page_maybe_dma_pinned could be further optimized down to one atomic in
non huge page cases.

Jason