From: Dave Hansen tl;dr: lock_vma_under_rcu() is already a trylock. No need to do both it and mmap_read_trylock(). Long Version: == Background == Historically, binder used an mmap_read_trylock() in its shrinker code. This ensures that reclaim is not blocked on an mmap_lock. Commit 95bc2d4a9020 ("binder: use per-vma lock in page reclaiming") added support for the per-VMA lock, but left mmap_read_trylock() as a fallback. This was presumably because the per-VMA locking can fail for several reasons and most (all?) lock_vma_under_rcu() callers have a fallback to mmap_read_trylock(). == Problem == The fallback is not worth the complexity here. lock_vma_under_rcu() is essentially already a non-blocking trylock. The main reason it fails is also the reason mmap_read_trylock() fails: something is holding mmap_write_lock(). The only remedy for a collision with mmap_write_lock() is to wait, which this code can not do. So the "fallback" after lock_vma_under_rcu() failure is not really a fallback: it is really likely to just be retrying in vain. That retry in an of itself isn't horrible. But it adds complexity. == Solution == Now that per-VMA locks are universally available, lock_vma_under_rcu() will not persistently fail. Rely on it alone and simplify the code. The removal of the fallback does not affect NOMMU case because binder driver depends on CONFIG_MMU. While at it we also make the handling of the cases where the original binder VMA is gone consistent. There are two cases to consider when Binder VMA is gone: 1. there is no VMA at that location anymore. 2. there is now another unrelated VMA at that location. Before this change we handle case 1 by having the shrinker proceed to free the page, and just skip the zap_vma_range() call. And we handle case 2 by having the shrinker return LRU_SKIP. While either behavior is acceptable, we need to handle them in a consistent way. Handle both cases by freeing the page without touching the VMA (skipping the zap_vma_range()). Full disclosure: I originally tried to do this with lock_vma_under_rcu_wait(), but it did not fit well with the mmap_lock trylock semantics. Claude caught this in a review and suggested the approach in this path. It seemed sane to me. So, Suggesed-by: Claude, I guess. Signed-off-by: Dave Hansen Cc: Andrew Morton Cc: Liam R. Howlett Cc: Vlastimil Babka Cc: Shakeel Butt Cc: linux-mm@kvack.org Cc: Greg Kroah-Hartman Cc: Arve Hjønnevåg Cc: Todd Kjos Cc: Christian Brauner Cc: Carlos Llamas Cc: Alice Ryhl Cc: David S. Miller Cc: David Ahern Cc: netdev@vger.kernel.org Acked-by: Lorenzo Stoakes (ARM) Reviewed-by: Alice Ryhl Signed-off-by: Suren Baghdasaryan --- drivers/android/binder_alloc.c | 45 ++++++++++++++++------------------ 1 file changed, 21 insertions(+), 24 deletions(-) diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c index e4488ad86a65..c13a588c37de 100644 --- a/drivers/android/binder_alloc.c +++ b/drivers/android/binder_alloc.c @@ -1142,7 +1142,6 @@ enum lru_status binder_alloc_free_page(struct list_head *item, struct vm_area_struct *vma; struct page *page_to_free; unsigned long page_addr; - int mm_locked = 0; size_t index; if (!mmget_not_zero(mm)) @@ -1151,27 +1150,25 @@ enum lru_status binder_alloc_free_page(struct list_head *item, index = mdata->page_index; page_addr = alloc->vm_start + index * PAGE_SIZE; - /* attempt per-vma lock first */ + /* + * Attempt per-vma lock. This is essentially a + * "trylock". It can fail even if the VMA exists + * for 'page_addr'. + */ vma = lock_vma_under_rcu(mm, page_addr); if (!vma) { - /* fall back to mmap_lock */ - if (!mmap_read_trylock(mm)) - goto err_mmap_read_lock_failed; - mm_locked = 1; - vma = vma_lookup(mm, page_addr); + /* + * If the vma exists, we can't continue because we cannot + * remove the page from the vma. However, if the vma was + * unmapped, it's okay to continue. + */ + if (binder_alloc_is_mapped(alloc)) + goto err_vma_lock_failed; } if (!mutex_trylock(&alloc->mutex)) goto err_get_alloc_mutex_failed; - /* - * Since a binder_alloc can only be mapped once, we ensure - * the vma corresponds to this mapping by checking whether - * the binder_alloc is still mapped. - */ - if (vma && !binder_alloc_is_mapped(alloc)) - goto err_invalid_vma; - trace_binder_unmap_kernel_start(alloc, index); page_to_free = alloc->pages[index]; @@ -1182,7 +1179,12 @@ enum lru_status binder_alloc_free_page(struct list_head *item, list_lru_isolate(lru, item); spin_unlock(&lru->lock); - if (vma) { + /* + * Since a binder_alloc can only be mapped once, we ensure + * the vma corresponds to this mapping by checking whether + * the binder_alloc is still mapped. + */ + if (vma && binder_alloc_is_mapped(alloc)) { trace_binder_unmap_user_start(alloc, index); zap_vma_range(vma, page_addr, PAGE_SIZE); @@ -1191,23 +1193,18 @@ enum lru_status binder_alloc_free_page(struct list_head *item, } mutex_unlock(&alloc->mutex); - if (mm_locked) - mmap_read_unlock(mm); - else + if (vma) vma_end_read(vma); mmput_async(mm); binder_free_page(page_to_free); return LRU_REMOVED_RETRY; -err_invalid_vma: mutex_unlock(&alloc->mutex); err_get_alloc_mutex_failed: - if (mm_locked) - mmap_read_unlock(mm); - else + if (vma) vma_end_read(vma); -err_mmap_read_lock_failed: +err_vma_lock_failed: mmput_async(mm); err_mmget: return LRU_SKIP; -- 2.55.0.691.gc56d675ccc-goog