Seems to me that the bug is in function
bdi_position_ratio()
within file
mm/page-writeback.c
The internal variable declaration is
long long pos_ratio;
and calculation of it overflows. - Maybe, changing the declaration to
u64 would help. But also, pos_ratio is used without any bounds checks
as return value though that is declared as unsigned long.
I do not yet understand what bdi_position_ratio() is meant to do, so
cannot yet offer patches.
---
What I did:
I added many lines like
BUG_ON(pos_ratio<0);
into kernel sources. Running that kernel and creating my files with
n=0; while [ $n -lt 99 ]; do dd bs=1M count=1024 if=/dev/zero of=x$n; (( n =
$n + 1 )); done &
I got after about 15 files created:
/bin/bash: line 1: 2755 Segmentation fault dd bs=1M count=1024
if=/dev/zero of=x$n
Message from syslogd@zeno at Sat Dec 15 19:46:37 2012 ...
zeno kernel: ------------[ cut here ]------------
zeno kernel: invalid opcode: 0000 [#1] SMP
...
and in the logs:
Dec 15 19:46:37 zeno kernel: ------------[ cut here ]------------
Dec 15 19:46:37 zeno kernel: kernel BUG at mm/page-writeback.c:569!
Dec 15 19:46:37 zeno kernel: invalid opcode: 0000 [#1] SMP
Dec 15 19:46:37 zeno kernel: Modules linked in: nfsd exportfs quota_v2
quota_tree fuse joydev usb_storage coretemp crc32c_intel aesni_intel sg cryptd
sr_mod aes_i586 aes_generic 8250_pnp evdev i2c_i801 8250 serial_core processor
thermal_sys button
Dec 15 19:46:37 zeno kernel:
Dec 15 19:46:37 zeno kernel: Pid: 2755, comm: dd Not tainted
3.2.32-pk06.08-i386t02 #1 Supermicro X9DR3-F/X9DR3-F
Dec 15 19:46:37 zeno kernel: EIP: 0060:[<c107bf30>] EFLAGS: 00010282 CPU: 0
Dec 15 19:46:37 zeno kernel: EIP is at bdi_position_ratio.isra.16+0x220/0x230
Dec 15 19:46:37 zeno kernel: EAX: fffaadbc EBX: 00000524 ECX: fffaadbc EDX:
760dae6b
Dec 15 19:46:37 zeno kernel: ESI: 00000524 EDI: ea673c18 EBP: d6235d2c ESP:
d6235d00
Dec 15 19:46:37 zeno kernel: DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Dec 15 19:46:37 zeno kernel: Process dd (pid: 2755, ti=d6234000 task=d607bb10
task.ti=d6234000)
Dec 15 19:46:37 zeno kernel: Stack:
Dec 15 19:46:37 zeno kernel: d6235d1c 000280cd 33b036ce 00000098 0000047f
760db26b fffaadbc 0000007a
Dec 15 19:46:37 zeno kernel: 00000004 00000546 d5de809c d6235db0 c107c963
000004f1 00000523 00000546
Dec 15 19:46:37 zeno kernel: 00000000 00140669 00000007 00000000 d5de80bc
00032afc 00000000 d5e83800
Dec 15 19:46:37 zeno kernel: Call Trace:
Dec 15 19:46:37 zeno kernel: [<c107c963>]
balance_dirty_pages_ratelimited_nr+0x253/0x520
Dec 15 19:46:37 zeno kernel: [<c10747cf>]
generic_file_buffered_write+0x16f/0x210
Dec 15 19:46:37 zeno kernel: [<c1075f7d>] __generic_file_aio_write+0x24d/0x4b0
Dec 15 19:46:37 zeno kernel: [<c1076240>] generic_file_aio_write+0x60/0xc0
Dec 15 19:46:37 zeno kernel: [<c10a2fa7>] do_sync_write+0xb7/0xf0
Dec 15 19:46:37 zeno kernel: [<c1036455>] ? irq_exit+0x55/0x60
Dec 15 19:46:37 zeno kernel: [<c10a2ef0>] ? wait_on_retry_sync_kiocb+0x50/0x50
Dec 15 19:46:37 zeno kernel: [<c10a3aa7>] vfs_write+0x87/0x170
Dec 15 19:46:37 zeno kernel: [<c10a2ef0>] ? wait_on_retry_sync_kiocb+0x50/0x50
Dec 15 19:46:37 zeno kernel: [<c10a3da8>] sys_write+0x38/0x70
Dec 15 19:46:37 zeno kernel: [<c160fd14>] sysenter_do_call+0x12/0x26
Dec 15 19:46:37 zeno kernel: Code: 55 ff ff ff 0f 0b 90 8d 74 26 00 0f a4 cb 03
c1 e1 03 e9 74 ff ff ff 8d 74 26 00 89 d0 31 d2 f7 75 10 89 c6 e9 59 ff ff ff
0f 0b <0f> 0b 0f 0b 0f 0b 31 c0 e9 5d ff ff ff 8d 76 00 55 89 e5 83 ec
Dec 15 19:46:37 zeno kernel: EIP: [<c107bf30>]
bdi_position_ratio.isra.16+0x220/0x230 SS:ESP 0068:d6235d00
Dec 15 19:46:37 zeno kernel: ---[ end trace c9c79e2ba8a36130 ]---
Relevant part of file mm/page-writeback.c :
525 static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
526 unsigned long thresh,
527 unsigned long bg_thresh,
528 unsigned long dirty,
529 unsigned long bdi_thresh,
530 unsigned long bdi_dirty)
531 {
532 unsigned long write_bw = bdi->avg_write_bandwidth;
533 unsigned long freerun = dirty_freerun_ceiling(thresh,
bg_thresh);
534 unsigned long limit = hard_dirty_limit(thresh);
535 unsigned long x_intercept;
536 unsigned long setpoint; /* dirty pages' target balance
point */
537 unsigned long bdi_setpoint;
538 unsigned long span;
539 long long pos_ratio; /* for scaling up/down the rate
limit */
540 long x;
541
542 if (unlikely(dirty >= limit))
543 return 0;
544
545 /*
546 * global setpoint
547 *
548 * setpoint - dirty 3
549 * f(dirty) := 1.0 + (----------------)
550 * limit - setpoint
551 *
552 * it's a 3rd order polynomial that subjects to
553 *
554 * (1) f(freerun) = 2.0 => rampup dirty_ratelimit reasonably
fast
555 * (2) f(setpoint) = 1.0 => the balance point
556 * (3) f(limit) = 0 => the hard limit
557 * (4) df/dx <= 0 => negative feedback control
558 * (5) the closer to setpoint, the smaller |df/dx| (and the
reverse)
559 * => fast response on large errors; small oscillation near
setpoint
560 */
561 setpoint = (freerun + limit) / 2;
562 x = div_s64((setpoint - dirty) << RATELIMIT_CALC_SHIFT,
563 limit - setpoint + 1);
564 BUG_ON(x<0);
565 pos_ratio = x;
566 pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
567 pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
568 pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
569 BUG_ON(pos_ratio<0);
570
571 /*
572 * We have computed basic pos_ratio above based on global
situation. If
573 * the bdi is over/under its share of dirty pages, we want to
scale
574 * pos_ratio further down/up. That is done by the following
mechanism.
575 */
576
577 /*
578 * bdi setpoint
579 *
580 * f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
581 *
582 * x_intercept - bdi_dirty
583 * := --------------------------
584 * x_intercept - bdi_setpoint
585 *
586 * The main bdi control line is a linear function that subjects
to
587 *
588 * (1) f(bdi_setpoint) = 1.0
589 * (2) k = - 1 / (8 * write_bw) (in single bdi case)
590 * or equally: x_intercept = bdi_setpoint + 8 * write_bw
591 *
592 * For single bdi case, the dirty pages are observed to
fluctuate
593 * regularly within range
594 * [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
595 * for various filesystems, where (2) can yield in a reasonable
12.5%
596 * fluctuation range for pos_ratio.
597 *
598 * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate
up to its
599 * own size, so move the slope over accordingly and choose a
slope that
600 * yields 100% pos_ratio fluctuation on suddenly doubled
bdi_thresh.
601 */
602 if (unlikely(bdi_thresh > thresh))
603 bdi_thresh = thresh;
604 /*
605 * It's very possible that bdi_thresh is close to 0 not because
the
606 * device is slow, but that it has remained inactive for long
time.
607 * Honour such devices a reasonable good (hopefully IO
efficient)
608 * threshold, so that the occasional writes won't be blocked
and active
609 * writes can rampup the threshold quickly.
610 */
611 bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
612 /*
613 * scale global setpoint to bdi's:
614 * bdi_setpoint = setpoint * bdi_thresh / thresh
615 */
616 x = div_u64((u64)bdi_thresh << 16, thresh + 1);
617 BUG_ON(x<0);
618 bdi_setpoint = setpoint * (u64)x >> 16;
619 /*
620 * Use span=(8*write_bw) in single bdi case as indicated by
621 * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD
case.
622 *
623 * bdi_thresh thresh - bdi_thresh
624 * span = ---------- * (8 * write_bw) + ------------------- *
bdi_thresh
625 * thresh thresh
626 */
627 span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
628 x_intercept = bdi_setpoint + span;
629
630 if (bdi_dirty < x_intercept - span / 4) {
631 pos_ratio = div_u64(pos_ratio * (x_intercept -
bdi_dirty),
632 x_intercept - bdi_setpoint + 1);
633 } else
634 pos_ratio /= 4;
635 BUG_ON(pos_ratio<0);
636
637 /*
638 * bdi reserve area, safeguard against dirty pool underrun and
disk idle
639 * It may push the desired control point of global dirty pages
higher
640 * than setpoint.
641 */
642 x_intercept = bdi_thresh / 2;
643 if (bdi_dirty < x_intercept) {
644 if (bdi_dirty > x_intercept / 8)
645 pos_ratio = div_u64(pos_ratio * x_intercept,
bdi_dirty);
646 else
647 pos_ratio *= 8;
648 }
649 BUG_ON(pos_ratio<0);
650
651 return pos_ratio;
652 }
Cheers, Paul
Paul Szabo p...@maths.usyd.edu.au http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics University of Sydney Australia
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