Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

From: John Hubbard
Date: Mon Sep 21 2020 - 19:53:41 EST

On 9/21/20 2:17 PM, Peter Xu wrote:
(Commit message collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by keeping

Not yet, it doesn't. :) More:

track if the mm_struct has ever been used with pin_user_pages(). mm_structs
that have never been passed to pin_user_pages() cannot have a positive
page_maybe_dma_pinned() by definition. This allows cases that might drive up
the page ref_count to avoid any penalty from handling dma_pinned pages.

Due to complexities with unpining this trivial version is a permanent sticky
bit, future work will be needed to make this a counter.

How about this instead:

Subsequent patches intend to reduce the chance of false positives from
page_maybe_dma_pinned(), by also considering whether or not a page has
even been part of an mm struct that has ever had pin_user_pages*()
applied to any of its pages.

In order to allow that, provide a boolean value (even though it's not
implemented exactly as a boolean type) within the mm struct, that is
simply set once and never cleared. This will suffice for an early, rough
implementation that fixes a few problems.

Future work is planned, to provide a more sophisticated solution, likely
involving a counter, and *not* involving something that is set and never

Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
include/linux/mm_types.h | 10 ++++++++++
kernel/fork.c | 1 +
mm/gup.c | 6 ++++++
3 files changed, 17 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 496c3ff97cce..6f291f8b74c6 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -441,6 +441,16 @@ struct mm_struct {
int map_count; /* number of VMAs */
+ /**
+ * @has_pinned: Whether this mm has pinned any pages. This can
+ * be either replaced in the future by @pinned_vm when it
+ * becomes stable, or grow into a counter on its own. We're
+ * aggresive on this bit now - even if the pinned pages were
+ * unpinned later on, we'll still keep this bit set for the
+ * lifecycle of this mm just for simplicity.
+ */
+ int has_pinned;

I think this would be elegant as an atomic_t, and using atomic_set() and
atomic_read(), which seem even more self-documenting that what you have here.

But it's admittedly a cosmetic point, combined with my perennial fear that
I'm missing something when I look at a READ_ONCE()/WRITE_ONCE() pair. :)

It's completely OK to just ignore this comment, but I didn't want to completely
miss the opportunity to make it a tiny bit cleaner to the reader.

John Hubbard