Re: [RFC][PATCH v2 1/5] mm: Avoid unmapping pinned pages

On Thu, Jan 20, 2022 at 10:03:44AM -0800, Nadav Amit wrote:

quoted

--- a/mm/mprotect.c
+++ b/mm/mprotect.c

@@ -106,6 +106,12 @@ static unsigned long change_pte_range(st

					continue;

				/*
+				 * Can't migrate pinned pages, avoid touching them.
+				 */
+				if (page_maybe_dma_pinned(page))
+					continue;
+
+				/*

I have a similar problem with userfaultfd changing protection for
DMA-pinned pages. For userfaultfd it is important to know how many
pages were actually modified.

I am working on a vectored UFFDIO_WRITEPROTECTV that aborts once
a pinned page is encountered, but also returns the number of pages
that were properly protected. I still need to do some work to
send patches for that as it requires further changes (to return
the number of pages that were handled).

But for the matter of your patch, is it possible to make this
test generic (not migration specific) and rely on a new flag in
cp_flags? I can of course make this change later if you prefer it
this way.

I have no objection to making it apply more widely, but I'm currently
only interested in the rmap users. If userspace does mprotect() on it's
own pages, it gets to keep whatever pieces are the result of that.