Re: [PATCH 1/2] mtd: ubi: block: forbid direct reclaim I/O during synchronous read

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2026/10/15 15:33, Yelin Fei 写道:
> From: Ye Linfei <yelinfei@ruijie.com.cn>
> 
> ubiblock implements ->queue_rq() synchronously: the task submitting the
> bio executes the whole UBI/MTD read itself, from inside the block layer
> submission loop (current->bio_list is non-empty) and while holding the
> tag_set SRCU read lock (blk_mq_run_dispatch_ops(), taken for
> BLK_MQ_F_BLOCKING drivers).
> 
> An allocation in that context may trigger direct reclaim, which with
> GFP_NOFS - used by all UBI allocations - may synchronously write dirty
> pages to swap.  On systems where swap is backed by a device with
> BLK_FEAT_SYNCHRONOUS (zram), that ends in swap_writepage_bdev_sync()
> -> submit_bio_wait().  The swap bio is only queued onto
> current->bio_list, never dispatched: per submit_bio_noacct_nocheck(),
> bios submitted while a submission loop is active are only collected on
> current->bio_list, and their sole dispatcher is the task currently
> executing ->queue_rq() - which is now sleeping waiting for the very
> same bio's completion.  The task hangs forever, the pending SRCU read
> lock freezes grace periods (blk_mq_timeout_work() -> synchronize_srcu()
> never returns), and the unfinished request blocks all other I/O to the
> device.
> 
> This was observed in the field on a 1 GiB RISC-V networking product
> running squashfs-on-ubiblock with zram swap: 47 tasks stuck in disk
> sleep for 600+ seconds, ending in a hung task panic.  The allocator
> never reached the OOM decision point, because the task died inside
> try_to_free_pages().
> 
> Mark the whole read with memalloc_noio_save()/memalloc_noio_restore()
> so direct reclaim never issues block I/O while this driver executes:
> reclaim is restricted to dropping clean page cache and shrinking slabs,
> page (swap) reclaim is performed asynchronously by kswapd, and the
> worst case is -ENOMEM, which propagates to the caller as an ordinary
> recoverable I/O error - a recoverable read failure instead of a total
> system hang.
> 
> This covers every allocation performed below ubi_read_sg(), including
> lock-tree entries, VID buffers and I/O helpers, regardless of the gfp
> flags they use internally.
> 
> Fixes: 9d54c8a33eec ("UBI: R/O block driver on top of UBI volumes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ye Linfei <yelinfei@ruijie.com.cn>
> ---
>   drivers/mtd/ubi/block.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)

Looks like other block drivers(eg. nvme/nbd/rbd..)  won't trigger direct 
mem reclaim by using flags 
GFP_ATOMIC(nvme_pci_setup_meta_iter)/GFP_NOIO(nbd 
sock_xmit)/GFP_NOWAIT(loop).

Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
> 
> diff --git a/drivers/mtd/ubi/block.c b/drivers/mtd/ubi/block.c
> index 29c0d6941a81..c9d42181bf64 100644
> --- a/drivers/mtd/ubi/block.c
> +++ b/drivers/mtd/ubi/block.c
> @@ -38,6 +38,7 @@
>   #include <linux/blkdev.h>
>   #include <linux/blk-mq.h>
>   #include <linux/hdreg.h>
> +#include <linux/sched/mm.h>
>   #include <linux/scatterlist.h>
>   #include <linux/idr.h>
>   #include <asm/div64.h>
> @@ -189,6 +190,7 @@ static blk_status_t ubiblock_read(struct request *req)
>   	int leb = pos;
>   	struct req_iterator iter;
>   	struct bio_vec bvec;
> +	unsigned int noio_flags;
>   	int ret;
>   
>   	blk_mq_start_request(req);
> @@ -201,6 +203,18 @@ static blk_status_t ubiblock_read(struct request *req)
>   	ubi_sgl_init(&pdu->usgl);
>   	blk_rq_map_sg(req, pdu->usgl.sg);
>   
> +	/*
> +	 * This entire callback runs in the synchronous dispatch path of
> +	 * blk-mq: submit_bio_noacct_nocheck() only queues a bio submitted
> +	 * from here onto current->bio_list, and the sole dispatcher of
> +	 * that list is this very task, busy inside ->queue_rq().  Any
> +	 * block I/O issued from below would therefore deadlock against
> +	 * itself (e.g. synchronous swap writeback reached via direct
> +	 * reclaim).  Forbid it for the whole read instead of relying on
> +	 * every callee using safe allocation flags.
> +	 */
> +	noio_flags = memalloc_noio_save();
> +
>   	while (bytes_left) {
>   		/*
>   		 * We can only read one LEB at a time. Therefore if the read
> @@ -221,6 +235,7 @@ static blk_status_t ubiblock_read(struct request *req)
>   
>   	rq_for_each_segment(bvec, req, iter)
>   		flush_dcache_page(bvec.bv_page);
> +	memalloc_noio_restore(noio_flags);
>   
>   	blk_mq_end_request(req, errno_to_blk_status(ret));
>   
>