The patch titled
Subject: mm: handle swap page faults under per-VMA lock
has been added to the -mm mm-unstable branch. Its filename is
mm-handle-swap-page-faults-under-per-vma-lock.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-handle-swap-page-faults-under-per-vma-lock.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: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Subject: mm: handle swap page faults under per-VMA lock
Date: Fri, 30 Jun 2023 14:19:56 -0700
When page fault is handled under per-VMA lock protection, all swap page
faults are retried with mmap_lock because folio_lock_or_retry has to drop
and reacquire mmap_lock if folio could not be immediately locked. Follow
the same pattern as mmap_lock to drop per-VMA lock when waiting for folio
and retrying once folio is available.
With this obstacle removed, enable do_swap_page to operate under per-VMA
lock protection. Drivers implementing ops->migrate_to_ram might still
rely on mmap_lock, therefore we have to fall back to mmap_lock in that
particular case.
Note that the only time do_swap_page calls synchronous swap_readpage is
when SWP_SYNCHRONOUS_IO is set, which is only set for
QUEUE_FLAG_SYNCHRONOUS devices: brd, zram and nvdimms (both btt and pmem).
Therefore we don't sleep in this path, and there's no need to drop the
mmap or per-VMA lock.
Link: https://lkml.kernel.org/r/20230630211957.1341547-6-surenb@xxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Tested-by: Alistair Popple <apopple@xxxxxxxxxx>
Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Michel Lespinasse <michel@xxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Punit Agrawal <punit.agrawal@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 13 +++++++++++++
mm/filemap.c | 17 ++++++++---------
mm/memory.c | 16 ++++++++++------
3 files changed, 31 insertions(+), 15 deletions(-)
--- a/include/linux/mm.h~mm-handle-swap-page-faults-under-per-vma-lock
+++ a/include/linux/mm.h
@@ -720,6 +720,14 @@ static inline void vma_mark_detached(str
vma->detached = detached;
}
+static inline void release_fault_lock(struct vm_fault *vmf)
+{
+ if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+ vma_end_read(vmf->vma);
+ else
+ mmap_read_unlock(vmf->vma->vm_mm);
+}
+
struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
unsigned long address);
@@ -735,6 +743,11 @@ static inline void vma_assert_write_lock
static inline void vma_mark_detached(struct vm_area_struct *vma,
bool detached) {}
+static inline void release_fault_lock(struct vm_fault *vmf)
+{
+ mmap_read_unlock(vmf->vma->vm_mm);
+}
+
#endif /* CONFIG_PER_VMA_LOCK */
/*
--- a/mm/filemap.c~mm-handle-swap-page-faults-under-per-vma-lock
+++ a/mm/filemap.c
@@ -1671,27 +1671,26 @@ static int __folio_lock_async(struct fol
* Return values:
* 0 - folio is locked.
* non-zero - folio is not locked.
- * mmap_lock has been released (mmap_read_unlock(), unless flags had both
- * FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in
- * which case mmap_lock is still held.
+ * mmap_lock or per-VMA lock has been released (mmap_read_unlock() or
+ * vma_end_read()), unless flags had both FAULT_FLAG_ALLOW_RETRY and
+ * FAULT_FLAG_RETRY_NOWAIT set, in which case the lock is still held.
*
* If neither ALLOW_RETRY nor KILLABLE are set, will always return 0
- * with the folio locked and the mmap_lock unperturbed.
+ * with the folio locked and the mmap_lock/per-VMA lock is left unperturbed.
*/
vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)
{
- struct mm_struct *mm = vmf->vma->vm_mm;
unsigned int flags = vmf->flags;
if (fault_flag_allow_retry_first(flags)) {
/*
- * CAUTION! In this case, mmap_lock is not released
- * even though return VM_FAULT_RETRY.
+ * CAUTION! In this case, mmap_lock/per-VMA lock is not
+ * released even though returning VM_FAULT_RETRY.
*/
if (flags & FAULT_FLAG_RETRY_NOWAIT)
return VM_FAULT_RETRY;
- mmap_read_unlock(mm);
+ release_fault_lock(vmf);
if (flags & FAULT_FLAG_KILLABLE)
folio_wait_locked_killable(folio);
else
@@ -1703,7 +1702,7 @@ vm_fault_t __folio_lock_or_retry(struct
ret = __folio_lock_killable(folio);
if (ret) {
- mmap_read_unlock(mm);
+ release_fault_lock(vmf);
return VM_FAULT_RETRY;
}
} else {
--- a/mm/memory.c~mm-handle-swap-page-faults-under-per-vma-lock
+++ a/mm/memory.c
@@ -3732,12 +3732,6 @@ vm_fault_t do_swap_page(struct vm_fault
if (!pte_unmap_same(vmf))
goto out;
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
- ret = VM_FAULT_RETRY;
- vma_end_read(vma);
- goto out;
- }
-
entry = pte_to_swp_entry(vmf->orig_pte);
if (unlikely(non_swap_entry(entry))) {
if (is_migration_entry(entry)) {
@@ -3747,6 +3741,16 @@ vm_fault_t do_swap_page(struct vm_fault
vmf->page = pfn_swap_entry_to_page(entry);
ret = remove_device_exclusive_entry(vmf);
} else if (is_device_private_entry(entry)) {
+ if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+ /*
+ * migrate_to_ram is not yet ready to operate
+ * under VMA lock.
+ */
+ vma_end_read(vma);
+ ret = VM_FAULT_RETRY;
+ goto out;
+ }
+
vmf->page = pfn_swap_entry_to_page(entry);
vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
vmf->address, &vmf->ptl);
_
Patches currently in -mm which might be from surenb@xxxxxxxxxx are
swap-remove-remnants-of-polling-from-read_swap_cache_async.patch
mm-add-missing-vm_fault_result_trace-name-for-vm_fault_completed.patch
mm-drop-per-vma-lock-when-returning-vm_fault_retry-or-vm_fault_completed.patch
mm-change-folio_lock_or_retry-to-use-vm_fault-directly.patch
mm-handle-swap-page-faults-under-per-vma-lock.patch
mm-handle-userfaults-under-vma-lock.patch
