[PATCH v5 5/6] mm: handle swap page faults under per-VMA lock

Suren Baghdasaryan
2023-06-28T17:26:44Z

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.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Peter Xu <peterx@redhat.com>
---
include/linux/mm.h | 13 +++++++++++++
mm/filemap.c | 17 ++++++++---------
mm/memory.c | 16 ++++++++++------
3 files changed, 31 insertions(+), 15 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fec149585985..bbaec479bf98 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -723,6 +723,14 @@ static inline void vma_mark_detached(struct vm_area_struct *vma, bool detached)
struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
unsigned long address);

+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);
+}
+
#else /* CONFIG_PER_VMA_LOCK */

static inline void vma_init_lock(struct vm_area_struct *vma) {}
@@ -736,6 +744,11 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma) {}
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 */

/*
diff --git a/mm/filemap.c b/mm/filemap.c
index 52bcf12dcdbf..d4d8f474e0c5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1703,27 +1703,26 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
* Return values:
* 0 - folio is locked.
* VM_FAULT_RETRY - 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
@@ -1735,7 +1734,7 @@ vm_fault_t __folio_lock_or_retry(struct folio *folio, struct vm_fault *vmf)

ret = __folio_lock_killable(folio);
if (ret) {
- mmap_read_unlock(mm);
+ release_fault_lock(vmf);
return VM_FAULT_RETRY;
}
} else {
diff --git a/mm/memory.c b/mm/memory.c
index 345080052003..4fb8ecfc6d13 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3712,12 +3712,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
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)) {
@@ -3727,6 +3721,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
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);