mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()

Patch series "userfaultfd move option", v6.

This patch series introduces UFFDIO_MOVE feature to userfaultfd, which has
long been implemented and maintained by Andrea in his local tree [1], but
was not upstreamed due to lack of use cases where this approach would be
better than allocating a new page and copying the contents.  Previous
upstraming attempts could be found at [6] and [7].

UFFDIO_COPY performs ~20% better than UFFDIO_MOVE when the application
needs pages to be allocated [2].  However, with UFFDIO_MOVE, if pages are
available (in userspace) for recycling, as is usually the case in heap
compaction algorithms, then we can avoid the page allocation and memcpy
(done by UFFDIO_COPY).  Also, since the pages are recycled in the
userspace, we avoid the need to release (via madvise) the pages back to
the kernel [3].  We see over 40% reduction (on a Google pixel 6 device) in
the compacting thread's completion time by using UFFDIO_MOVE vs.
UFFDIO_COPY.  This was measured using a benchmark that emulates a heap
compaction implementation using userfaultfd (to allow concurrent accesses
by application threads).  More details of the usecase are explained in
[3].

Furthermore, UFFDIO_MOVE enables moving swapped-out pages without
touching them within the same vma. Today, it can only be done by mremap,
however it forces splitting the vma.

TODOs for follow-up improvements:
- cross-mm support. Known differences from single-mm and missing pieces:
	- memcg recharging (might need to isolate pages in the process)
	- mm counters
	- cross-mm deposit table moves
	- cross-mm test
	- document the address space where src and dest reside in struct
	  uffdio_move

- TLB flush batching.  Will require extensive changes to PTL locking in
  move_pages_pte().  OTOH that might let us reuse parts of mremap code.

This patch (of 5):

For now, folio_move_anon_rmap() was only used to move a folio to a
different anon_vma after fork(), whereby the root anon_vma stayed
unchanged.  For that, it was sufficient to hold the folio lock when
calling folio_move_anon_rmap().

However, we want to make use of folio_move_anon_rmap() to move folios
between VMAs that have a different root anon_vma.  As folio_referenced()
performs an RMAP walk without holding the folio lock but only holding the
anon_vma in read mode, holding the folio lock is insufficient.

When moving to an anon_vma with a different root anon_vma, we'll have to
hold both, the folio lock and the anon_vma lock in write mode.
Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
read-lock the anon_vma, we have to re-check if the mapping was changed in
the meantime.  If that was the case, we have to retry.

Note that folio_move_anon_rmap() must only be called if the anon page is
exclusive to a process, and must not be called on KSM folios.

This is a preparation for UFFDIO_MOVE, which will hold the folio lock, the
anon_vma lock in write mode, and the mmap_lock in read mode.

Link: https://lkml.kernel.org/r/20231206103702.3873743-1-surenb@google.com
Link: https://lkml.kernel.org/r/20231206103702.3873743-2-surenb@google.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: kernel-team@android.com
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index b086dc957b0cba054903db02efb9cbf173897139..15a55304aa3bfb42f7619d23d398675b43af1e7e 100644 (file)
--- 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;
 
+retry:
        rcu_read_lock();
        anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
@@ -552,6 +553,17 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
        anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
        root_anon_vma = READ_ONCE(anon_vma->root);
        if (down_read_trylock(&root_anon_vma->rwsem)) {
+               /*
+                * folio_move_anon_rmap() might have changed the anon_vma as we
+                * might not hold the folio lock here.
+                */
+               if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
+                            anon_mapping)) {
+                       up_read(&root_anon_vma->rwsem);
+                       rcu_read_unlock();
+                       goto retry;
+               }
+
                /*
                 * If the folio is still mapped, then this anon_vma is still
                 * its anon_vma, and holding the mutex ensures that it will
@@ -586,6 +598,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
        rcu_read_unlock();
        anon_vma_lock_read(anon_vma);
 
+       /*
+        * folio_move_anon_rmap() might have changed the anon_vma as we might
+        * not hold the folio lock here.
+        */
+       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 retry;
+       }
+
        if (atomic_dec_and_test(&anon_vma->refcount)) {
                /*
                 * Oops, we held the last refcount, release the lock