David Hildenbrand [off-list ref] writes:

On 27.06.24 02:54, Alistair Popple wrote:

quoted

Longterm pinning of FS DAX pages should already be disallowed by
various pXX_devmap checks. However a future change will cause these
checks to be invalid for FS DAX pages so make
folio_is_longterm_pinnable() return false for FS DAX pages.
Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
  include/linux/memremap.h | 11 +++++++++++
  include/linux/mm.h       |  4 ++++
  2 files changed, 15 insertions(+)

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 6505713..19a448e 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h

@@ -193,6 +193,17 @@ static inline bool folio_is_device_coherent(const struct folio *folio)
  	return is_device_coherent_page(&folio->page);
  }
  +static inline bool is_device_dax_page(const struct page *page)
+{
+	return is_zone_device_page(page) &&
+		page_dev_pagemap(page)->type == MEMORY_DEVICE_FS_DAX;
+}
+
+static inline bool folio_is_device_dax(const struct folio *folio)
+{
+	return is_device_dax_page(&folio->page);
+}
+
  #ifdef CONFIG_ZONE_DEVICE
  void zone_device_page_init(struct page *page);
  void *memremap_pages(struct dev_pagemap *pgmap, int nid);

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b84368b..4d1cdea 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -2032,6 +2032,10 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
  	if (folio_is_device_coherent(folio))
  		return false;
  +	/* DAX must also always allow eviction. */
+	if (folio_is_device_dax(folio))
+		return false;
+
  	/* Otherwise, non-movable zone folios can be pinned. */
  	return !folio_is_zone_movable(folio);
  

Why is the check in check_vma_flags() insufficient? GUP-fast maybe?

Right. This came up when I was changing the code for GUP-fast, but also
they shouldn't be longterm pinnable and adding the case to
folio_is_longterm_pinnable() is an excellent way of documenting that.