Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [High] Block I/O Forward Progress / Deadlock due to missing mempool backing for flush allocations --
commit e290d67d3c09c47f258d895e86c79743d39db2c1 Author: Li Chen <[email protected]> nvdimm: virtio_pmem: stop allocating child flush bio This commit avoids allocating a child PREFLUSH bio during REQ_FUA submission by queuing an ordered WQ_MEM_RECLAIM work item. It allows provider flush callbacks to return NVDIMM_FLUSH_ASYNC after taking ownership of parent bio completion. > diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c > index 4176046627beb..8e16b7780be1a 100644 > --- a/drivers/nvdimm/nd_virtio.c > +++ b/drivers/nvdimm/nd_virtio.c [ ... ] > @@ -107,30 +113,46 @@ static int virtio_pmem_flush(struct nd_region > *nd_region) > return err; > }; > > +static void virtio_pmem_flush_work(struct work_struct *work) > +{ > + struct virtio_pmem_flush_work *flush; > + int err; > + > + flush = container_of(work, struct virtio_pmem_flush_work, work); > + err = virtio_pmem_flush(flush->nd_region); [ ... ] > int async_pmem_flush(struct nd_region *nd_region, struct bio *bio) > { > - /* > - * Create child bio for asynchronous flush and chain with > - * parent bio. Otherwise directly call nd_region flush. > - */ > - if (bio && bio->bi_iter.bi_sector != -1) { > - struct bio *child = bio_alloc(bio->bi_bdev, 0, > - REQ_OP_WRITE | REQ_PREFLUSH, > - GFP_ATOMIC); > + struct virtio_device *vdev = nd_region->provider_data; > + struct virtio_pmem *vpmem = vdev->priv; > + struct virtio_pmem_flush_work *flush; > + int err; > > - if (!child) > + if (bio && bio->bi_iter.bi_sector != -1) { > + flush = kmalloc_obj(*flush, GFP_NOIO); [Severity: High] Can this cause a deadlock during memory reclaim? Order-0 allocations with GFP_NOIO (which includes __GFP_DIRECT_RECLAIM) can loop indefinitely in the page allocator's slowpath rather than failing. Since this lacks mempool backing and is directly in the block I/O submission path, couldn't the block submission thread block waiting for memory, deadlocking if memory cannot be freed without this block I/O completing? > + if (!flush) > return -ENOMEM; > - bio_clone_blkg_association(child, bio); > - child->bi_iter.bi_sector = -1; > - bio_chain(child, bio); > - submit_bio(child); > - return 0; > + > + INIT_WORK(&flush->work, virtio_pmem_flush_work); > + flush->nd_region = nd_region; > + flush->bio = bio; > + queue_work(vpmem->flush_wq, &flush->work); [Severity: High] Is it safe to queue work on a WQ_MEM_RECLAIM workqueue when the worker function performs a non-mempool-backed GFP_NOIO allocation? If the system is under severe memory pressure, the workqueue falls back to its rescuer thread to ensure forward progress. The rescuer executes virtio_pmem_flush_work() -> virtio_pmem_flush(), which allocates memory: drivers/nvdimm/nd_virtio.c:virtio_pmem_flush() { ... req_data = kmalloc_obj(*req_data, GFP_NOIO); ... } Since GFP_NOIO includes __GFP_DIRECT_RECLAIM, the rescuer thread could block indefinitely in direct reclaim trying to allocate req_data. Doesn't this stall the rescuer thread and prevent pending flush completions from making forward progress? > + return NVDIMM_FLUSH_ASYNC; > } -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=4
