On Wed, Jun 28, 2023 at 12:17:59AM -0700, Suren Baghdasaryan wrote:
> 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@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>
One nit below:
> ---
> mm/filemap.c | 25 ++++++++++++++++---------
> mm/memory.c | 16 ++++++++++------
> 2 files changed, 26 insertions(+), 15 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 52bcf12dcdbf..7ee078e1a0d2 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1699,31 +1699,38 @@ static int __folio_lock_async(struct folio *folio, struct wait_page_queue *wait)
> return ret;
> }
>
> +static 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);
> +}
> +
> /*
> * 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 +1742,7 @@ 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..76c7907e7286 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;
Here IIUC ret==0 is guaranteed, so maybe "ret = VM_FAULT_RETRY" is slightly
clearer.
> + 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);
> --
> 2.41.0.162.gfafddb0af9-goog
>
--
Peter Xu
