Re: [PATCH v8] bio: limit bio max size
From: Marek Szyprowski <m.szyprowski@samsung.com>
Date: 2021-04-26 13:18:25
Also in:
lkml
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 <redacted>
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.
quoted hunk ↗ jump to hunk
--- 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