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

Jason Gunthorpe
2020-09-16T19:02:06Z

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