linux-kernel - Re: [PATCH v2] mm: re-allow pinning of zero pfns (again)

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Message-Id: <20220827184021.4e100b9eda3a9141217ec60d@linux-foundation.org>
Date:   Sat, 27 Aug 2022 18:40:21 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, alex.sierra@....com,
        willy@...radead.org, hch@....de, Felix.Kuehling@....com,
        apopple@...dia.com, david@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: re-allow pinning of zero pfns (again)

On Sat, 27 Aug 2022 17:59:32 -0700 John Hubbard <jhubbard@...dia.com> wrote:

> 
> 	/* The zero page is always allowed to be pinned. */

Wow, that's really verbose :)

--- a/include/linux/mm.h~mm-re-allow-pinning-of-zero-pfns-again-fix
+++ a/include/linux/mm.h
@@ -1544,11 +1544,7 @@ static inline bool is_longterm_pinnable_
 	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
 		return false;
 #endif
-	/*
-	 * The zero page might reside in a movable zone, however it may not
-	 * be migrated and can therefore be pinned.  The vfio subsystem pins
-	 * user mappings including the zero page for IOMMU translation.
-	 */
+	/* The zero page may always be pinned */
 	if (is_zero_pfn(page_to_pfn(page)))
 		return true;
 
_