Re: [PATCH v8] bio: limit bio max size

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Changheu,

On 21.04.2021 11:47, Changheun Lee wrote:
> bio size can grow up to 4GB when muli-page bvec is enabled.
> but sometimes it would lead to inefficient behaviors.
> in case of large chunk direct I/O, - 32MB chunk read in user space -
> all pages for 32MB would be merged to a bio structure if the pages
> physical addresses are contiguous. it makes some delay to submit
> until merge complete. bio max size should be limited to a proper size.
>
> When 32MB chunk read with direct I/O option is coming from userspace,
> kernel behavior is below now in do_direct_IO() loop. it's timeline.
>
>   | bio merge for 32MB. total 8,192 pages are merged.
>   | total elapsed time is over 2ms.
>   |------------------ ... ----------------------->|
>                                                   | 8,192 pages merged a bio.
>                                                   | at this time, first bio submit is done.
>                                                   | 1 bio is split to 32 read request and issue.
>                                                   |--------------->
>                                                    |--------------->
>                                                     |--------------->
>                                                                ......
>                                                                     |--------------->
>                                                                      |--------------->|
>                            total 19ms elapsed to complete 32MB read done from device. |
>
> If bio max size is limited with 1MB, behavior is changed below.
>
>   | bio merge for 1MB. 256 pages are merged for each bio.
>   | total 32 bio will be made.
>   | total elapsed time is over 2ms. it's same.
>   | but, first bio submit timing is fast. about 100us.
>   |--->|--->|--->|---> ... -->|--->|--->|--->|--->|
>        | 256 pages merged a bio.
>        | at this time, first bio submit is done.
>        | and 1 read request is issued for 1 bio.
>        |--------------->
>             |--------------->
>                  |--------------->
>                                        ......
>                                                   |--------------->
>                                                    |--------------->|
>          total 17ms elapsed to complete 32MB read done from device. |
>
> As a result, read request issue timing is faster if bio max size is limited.
> Current kernel behavior with multipage bvec, super large bio can be created.
> And it lead to delay first I/O request issue.
>
> Signed-off-by: Changheun Lee <nanich.lee@samsung.com>

This patch landed in linux-next 20210426 as commit 42fb54fbc707 ("bio: 
limit bio max size"). Sadly it causes the following regression during 
boot on my test systems:

8<--- cut here ---
Unable to handle kernel NULL pointer dereference at virtual address 0000023c
pgd = (ptrval)
[0000023c] *pgd=00000000
Internal error: Oops: 5 [#2] SMP ARM
Modules linked in:
CPU: 0 PID: 186 Comm: systemd-udevd Tainted: G      D 
5.12.0-next-20210426 #3045
Hardware name: Generic DT based system
PC is at bio_add_hw_page+0x58/0x1fc
LR is at bio_add_pc_page+0x40/0x5c
pc : [<c06c5bf0>]    lr : [<c06c5dd4>]    psr: 20000013
sp : c3cc7de0  ip : ffffffff  fp : 00000000
r10: 00000cc0  r9 : c20b2000  r8 : c21b5680
r7 : dbc51b80  r6 : c30d0540  r5 : 00000014  r4 : c21b5680
r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : c30d0540
Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 10c5387d  Table: 43ccc06a  DAC: 00000051
Register r0 information: slab request_queue start c30d0540 pointer offset 0
Register r1 information: NULL pointer
Register r2 information: NULL pointer
Register r3 information: NULL pointer
Register r4 information: slab kmalloc-128 start c21b5680 pointer offset 
0 size 128
Register r5 information: non-paged memory
Register r6 information: slab request_queue start c30d0540 pointer offset 0
Register r7 information: non-slab/vmalloc memory
Register r8 information: slab kmalloc-128 start c21b5680 pointer offset 
0 size 128
Register r9 information: slab kmalloc-4k start c20b2000 pointer offset 0 
size 4096
Register r10 information: non-paged memory
Register r11 information: NULL pointer
Register r12 information: non-paged memory
Process systemd-udevd (pid: 186, stack limit = 0x(ptrval))
Stack: (0xc3cc7de0 to 0xc3cc8000)
...
[<c06c5bf0>] (bio_add_hw_page) from [<c06c5dd4>] (bio_add_pc_page+0x40/0x5c)
[<c06c5dd4>] (bio_add_pc_page) from [<c06cf0ac>] 
(blk_rq_map_kern+0x234/0x304)
[<c06cf0ac>] (blk_rq_map_kern) from [<c0a54634>] (serial_show+0x64/0xd4)
[<c0a54634>] (serial_show) from [<c0a228ac>] (dev_attr_show+0x18/0x48)
[<c0a228ac>] (dev_attr_show) from [<c054721c>] (sysfs_kf_seq_show+0x88/0xf4)
[<c054721c>] (sysfs_kf_seq_show) from [<c04d7a44>] 
(seq_read_iter+0x10c/0x4bc)
[<c04d7a44>] (seq_read_iter) from [<c04adf60>] (vfs_read+0x1d4/0x2e0)
[<c04adf60>] (vfs_read) from [<c04ae47c>] (ksys_read+0x5c/0xd0)
[<c04ae47c>] (ksys_read) from [<c03000c0>] (ret_fast_syscall+0x0/0x58)
Exception stack(0xc3cc7fa8 to 0xc3cc7ff0)
...
Code: e1520003 9a000021 e5942004 e5941020 (e592223c)
---[ end trace 51c4d8003ec70244 ]---

It can be also reproduced with qemu and ARM 32bit virt machine. 
Reverting it on top of linux-next 20210426 fixes the issue. If you need 
more information, let me know.

> ---
>   block/bio.c            | 9 ++++++++-
>   block/blk-settings.c   | 5 +++++
>   include/linux/bio.h    | 4 +++-
>   include/linux/blkdev.h | 2 ++
>   4 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 50e579088aca..9e5061ecc317 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -255,6 +255,13 @@ void bio_init(struct bio *bio, struct bio_vec *table,
>   }
>   EXPORT_SYMBOL(bio_init);
>   
> +unsigned int bio_max_size(struct bio *bio)
> +{
> +	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
> +
> +	return q->limits.bio_max_bytes;
> +}
> +
>   /**
>    * bio_reset - reinitialize a bio
>    * @bio:	bio to reset
> @@ -866,7 +873,7 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>   		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
>   
>   		if (page_is_mergeable(bv, page, len, off, same_page)) {
> -			if (bio->bi_iter.bi_size > UINT_MAX - len) {
> +			if (bio->bi_iter.bi_size > bio_max_size(bio) - len) {
>   				*same_page = false;
>   				return false;
>   			}
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index b4aa2f37fab6..cd3dcb5afe50 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -37,6 +37,7 @@ EXPORT_SYMBOL_GPL(blk_queue_rq_timeout);
>    */
>   void blk_set_default_limits(struct queue_limits *lim)
>   {
> +	lim->bio_max_bytes = UINT_MAX;
>   	lim->max_segments = BLK_MAX_SEGMENTS;
>   	lim->max_discard_segments = 1;
>   	lim->max_integrity_segments = 0;
> @@ -168,6 +169,10 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
>   				 limits->logical_block_size >> SECTOR_SHIFT);
>   	limits->max_sectors = max_sectors;
>   
> +	if (check_shl_overflow(max_sectors, SECTOR_SHIFT,
> +				&limits->bio_max_bytes))
> +		limits->bio_max_bytes = UINT_MAX;
> +
>   	q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
>   }
>   EXPORT_SYMBOL(blk_queue_max_hw_sectors);
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index d0246c92a6e8..e5add63da3af 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -106,6 +106,8 @@ static inline void *bio_data(struct bio *bio)
>   	return NULL;
>   }
>   
> +extern unsigned int bio_max_size(struct bio *bio);
> +
>   /**
>    * bio_full - check if the bio is full
>    * @bio:	bio to check
> @@ -119,7 +121,7 @@ static inline bool bio_full(struct bio *bio, unsigned len)
>   	if (bio->bi_vcnt >= bio->bi_max_vecs)
>   		return true;
>   
> -	if (bio->bi_iter.bi_size > UINT_MAX - len)
> +	if (bio->bi_iter.bi_size > bio_max_size(bio) - len)
>   		return true;
>   
>   	return false;
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 158aefae1030..c205d60ac611 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -312,6 +312,8 @@ enum blk_zoned_model {
>   };
>   
>   struct queue_limits {
> +	unsigned int		bio_max_bytes;
> +
>   	unsigned long		bounce_pfn;
>   	unsigned long		seg_boundary_mask;
>   	unsigned long		virt_boundary_mask;

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland