[PATCH v2 1/3] userfaultfd: UFFDIO_REMAP: rmap preparation

Suren Baghdasaryan
2023-09-23T01:32:08Z

From: Andrea Arcangeli <aarcange@redhat.com>

As far as the rmap code is concerned, UFFDIO_REMAP only alters the
page->mapping and page->index. It does it while holding the page
lock. However folio_referenced() is doing rmap walks without taking the
folio lock first, so folio_lock_anon_vma_read() must be updated to
re-check that the folio->mapping didn't change after we obtained the
anon_vma read lock.

UFFDIO_REMAP takes the anon_vma lock for writing before altering the
folio->mapping, so if the folio->mapping is still the same after
obtaining the anon_vma read lock (without the folio lock), the rmap
walks can go ahead safely (and UFFDIO_REMAP will wait the rmap walk to
complete before proceeding).

UFFDIO_REMAP serializes against itself with the folio lock.

All other places taking the anon_vma lock while holding the mmap_lock
for writing, don't need to check if the folio->mapping has changed
after taking the anon_vma lock, regardless of the folio lock, because
UFFDIO_REMAP holds the mmap_lock for reading.

There's one constraint enforced to allow this simplification: the
source pages passed to UFFDIO_REMAP must be mapped only in one vma,
but this constraint is an acceptable tradeoff for UFFDIO_REMAP
users.

The source addresses passed to UFFDIO_REMAP can be set as
VM_DONTCOPY with MADV_DONTFORK to avoid any risk of the mapcount of
the pages increasing if some thread of the process forks() before
UFFDIO_REMAP run.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
mm/rmap.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index ec7f8e6c9e48..c1ebbd23fa61 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -542,6 +542,7 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
struct anon_vma *root_anon_vma;
unsigned long anon_mapping;

+repeat:
rcu_read_lock();
anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
@@ -586,6 +587,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
rcu_read_unlock();
anon_vma_lock_read(anon_vma);

+ /*
+ * Check if UFFDIO_REMAP changed the anon_vma. This is needed
+ * because we don't assume the folio was locked.
+ */
+ if (unlikely((unsigned long) READ_ONCE(folio->mapping) !=
+ anon_mapping)) {
+ anon_vma_unlock_read(anon_vma);
+ put_anon_vma(anon_vma);
+ anon_vma = NULL;
+ goto repeat;
+ }
+
if (atomic_dec_and_test(&anon_vma->refcount)) {
/*
* Oops, we held the last refcount, release the lock
--
2.42.0.515.g380fc7ccd1-goog