[PATCH v2 3/3] mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()

David Hildenbrand
2024-03-26T14:32:47Z

folio_is_secretmem() is currently only used during GUP-fast. Nowadays,
folio_fast_pin_allowed() performs similar checks during GUP-fast and
contains a lot of careful handling -- READ_ONCE() -- , sanity checks --
lockdep_assert_irqs_disabled() -- and helpful comments on how this
handling is safe and correct.

So let's merge folio_is_secretmem() into folio_fast_pin_allowed(). Rename
folio_fast_pin_allowed() to gup_fast_folio_allowed(), to better match the
new semantics.

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/secretmem.h | 21 ++---------------
mm/gup.c | 48 +++++++++++++++++++++++----------------
2 files changed, 30 insertions(+), 39 deletions(-)

diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
index acf7e1a3f3de..e918f96881f5 100644
--- a/include/linux/secretmem.h
+++ b/include/linux/secretmem.h
@@ -6,25 +6,8 @@

extern const struct address_space_operations secretmem_aops;

-static inline bool folio_is_secretmem(struct folio *folio)
+static inline bool secretmem_mapping(struct address_space *mapping)
{
- struct address_space *mapping;
-
- /*
- * Using folio_mapping() is quite slow because of the actual call
- * instruction.
- * We know that secretmem pages are not compound, so we can
- * save a couple of cycles here.
- */
- if (folio_test_large(folio))
- return false;
-
- mapping = (struct address_space *)
- ((unsigned long)folio->mapping & ~PAGE_MAPPING_FLAGS);
-
- if (!mapping || mapping != folio->mapping)
- return false;
-
return mapping->a_ops == &secretmem_aops;
}

@@ -38,7 +21,7 @@ static inline bool vma_is_secretmem(struct vm_area_struct *vma)
return false;
}

-static inline bool folio_is_secretmem(struct folio *folio)
+static inline bool secretmem_mapping(struct address_space *mapping)
{
return false;
}
diff --git a/mm/gup.c b/mm/gup.c
index e7510b6ce765..03b74b148e30 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2466,12 +2466,14 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
#ifdef CONFIG_HAVE_FAST_GUP

/*
- * Used in the GUP-fast path to determine whether a pin is permitted for a
- * specific folio.
+ * Used in the GUP-fast path to determine whether GUP is permitted to work on
+ * a specific folio.
*
* This call assumes the caller has pinned the folio, that the lowest page table
* level still points to this folio, and that interrupts have been disabled.
*
+ * GUP-fast must reject all secretmem folios.
+ *
* Writing to pinned file-backed dirty tracked folios is inherently problematic
* (see comment describing the writable_file_mapping_allowed() function). We
* therefore try to avoid the most egregious case of a long-term mapping doing
@@ -2481,25 +2483,34 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
* in the fast path, so instead we whitelist known good cases and if in doubt,
* fall back to the slow path.
*/
-static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
+static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
{
+ bool reject_file_backed = false;
struct address_space *mapping;
+ bool check_secretmem = false;
unsigned long mapping_flags;

/*
* If we aren't pinning then no problematic write can occur. A long term
* pin is the most egregious case so this is the one we disallow.
*/
- if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
+ if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) ==
(FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
- return true;
+ reject_file_backed = true;
+
+ /* We hold a folio reference, so we can safely access folio fields. */

- /* The folio is pinned, so we can safely access folio fields. */
+ /* secretmem folios are always order-0 folios. */
+ if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
+ check_secretmem = true;
+
+ if (!reject_file_backed && !check_secretmem)
+ return true;

if (WARN_ON_ONCE(folio_test_slab(folio)))
return false;

- /* hugetlb mappings do not require dirty-tracking. */
+ /* hugetlb neither requires dirty-tracking nor can be secretmem. */
if (folio_test_hugetlb(folio))
return true;

@@ -2535,10 +2546,12 @@ static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)

/*
* At this point, we know the mapping is non-null and points to an
- * address_space object. The only remaining whitelisted file system is
- * shmem.
+ * address_space object.
*/
- return shmem_mapping(mapping);
+ if (check_secretmem && secretmem_mapping(mapping))
+ return false;
+ /* The only remaining allowed file system is shmem. */
+ return !reject_file_backed || shmem_mapping(mapping);
}

static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
@@ -2624,18 +2637,13 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
if (!folio)
goto pte_unmap;

- if (unlikely(folio_is_secretmem(folio))) {
- gup_put_folio(folio, 1, flags);
- goto pte_unmap;
- }
-
if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
gup_put_folio(folio, 1, flags);
goto pte_unmap;
}

- if (!folio_fast_pin_allowed(folio, flags)) {
+ if (!gup_fast_folio_allowed(folio, flags)) {
gup_put_folio(folio, 1, flags);
goto pte_unmap;
}
@@ -2832,7 +2840,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
return 0;
}

- if (!folio_fast_pin_allowed(folio, flags)) {
+ if (!gup_fast_folio_allowed(folio, flags)) {
gup_put_folio(folio, refs, flags);
return 0;
}
@@ -2903,7 +2911,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
return 0;
}

- if (!folio_fast_pin_allowed(folio, flags)) {
+ if (!gup_fast_folio_allowed(folio, flags)) {
gup_put_folio(folio, refs, flags);
return 0;
}
@@ -2947,7 +2955,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
return 0;
}

- if (!folio_fast_pin_allowed(folio, flags)) {
+ if (!gup_fast_folio_allowed(folio, flags)) {
gup_put_folio(folio, refs, flags);
return 0;
}
@@ -2992,7 +3000,7 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
return 0;
}

- if (!folio_fast_pin_allowed(folio, flags)) {
+ if (!gup_fast_folio_allowed(folio, flags)) {
gup_put_folio(folio, refs, flags);
return 0;
}
--
2.43.2