Re: [PATCH 09/13] gup: Don't allow FOLL_LONGTERM pinning of FS DAX pages

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On 02.07.24 01:47, Alistair Popple wrote:

David Hildenbrand <david@xxxxxxxxxx> writes:

On 27.06.24 02:54, Alistair Popple wrote:
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@xxxxxxxxxx>
---
   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.

Makes sense, might be worth adding that (GUP-fast and check_vma_flags() covering GUP-slow) to the patch description.

--
Cheers,

David / dhildenb





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