[PATCH RFC gmem v1 1/8] mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory

[Michael Roth <michael.roth@amd.com>]

filemap users like guest_memfd may use page cache pages to
allocate/manage memory that is only intended to be accessed by guests
via hardware protections like encryption. Writes to memory of this sort
in common paths like truncation may cause unexpected behavior such
writing garbage instead of zeros when attempting to zero pages, or
worse, triggering hardware protections that are considered fatal as far
as the kernel is concerned.

Introduce a new address_space flag, AS_INACCESSIBLE, and use this
initially to prevent zero'ing of pages during truncation, with the
understanding that it is up to the owner of the mapping to handle this
specially if needed.

Link: https://lore.kernel.org/lkml/ZR9LYhpxTaTk6PJX@google.com/
Cc: Matthew Wilcox <willy@infradead.org>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 include/linux/pagemap.h | 1 +
 mm/truncate.c           | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 82c9bf506b79..9e79cf48f67a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -205,6 +205,7 @@ enum mapping_flags {
 	AS_LARGE_FOLIO_SUPPORT = 6,
 	AS_RELEASE_ALWAYS = 7,	/* Call ->release_folio(), even if no private data */
 	AS_UNMOVABLE	= 8,	/* The mapping cannot be moved, ever */
+	AS_INACCESSIBLE	= 9,	/* Do not attempt direct R/W access to the mapping */
 };
 
 /**
diff --git a/mm/truncate.c b/mm/truncate.c
index 8e3aa9e8618e..0d80bcc250af 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -233,7 +233,8 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 	 * doing a complex calculation here, and then doing the zeroing
 	 * anyway if the page split fails.
 	 */
-	folio_zero_range(folio, offset, length);
+	if (!(folio->mapping->flags & AS_INACCESSIBLE))
+		folio_zero_range(folio, offset, length);
 
 	if (folio_has_private(folio))
 		folio_invalidate(folio, offset, length);
-- 
2.25.1