The patch titled
Subject: mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()
has been added to the -mm mm-unstable branch. Its filename is
mm-rmap-support-move-to-different-root-anon_vma-in-folio_move_anon_rmap.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-rmap-support-move-to-different-root-anon_vma-in-folio_move_anon_rmap.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()
Date: Wed, 6 Dec 2023 02:36:55 -0800
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@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20231206103702.3873743-2-surenb@xxxxxxxxxx
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Brian Geffon <bgeffon@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Cc: Nicolas Geoffray <ngeoffray@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: ZhangPeng <zhangpeng362@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/rmap.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
--- a/mm/rmap.c~mm-rmap-support-move-to-different-root-anon_vma-in-folio_move_anon_rmap
+++ a/mm/rmap.c
@@ -542,6 +542,7 @@ struct anon_vma *folio_lock_anon_vma_rea
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)
@@ -553,6 +554,17 @@ struct anon_vma *folio_lock_anon_vma_rea
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
* not go away, see anon_vma_free().
@@ -586,6 +598,18 @@ struct anon_vma *folio_lock_anon_vma_rea
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
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
mm-rmap-support-move-to-different-root-anon_vma-in-folio_move_anon_rmap.patch
userfaultfd-uffdio_move-uabi.patch
