Re: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments

From: Nikanth Karthikesan
Date: Thu Oct 02 2008 - 11:06:20 EST

On Thursday 02 October 2008 19:59:33 Nikanth Karthikesan wrote:
> This is a follow-up to my earlier mail http://lkml.org/lkml/2008/9/23/294
> ([PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly)
>
> It is possible for the merging code to create lesser no of phys segments
> than hw segments, but every hw segment needs atleast one new phys segment.
> This triggers the BUG() on scsi_init_sgtable() as blk_rq_map_sg() returns
> more no of segments than rq->nr_phys_segments
>
> The following blktrace shows a sequence of bio's to trigger such condition
> on my machine with max_sectors_kb=512 & max_hw_sectors_kb=32767.
>
> 8,0 0 12 1.943680621 2261 Q R 6184075 + 55 [bash]
> 8,0 0 13 1.943684671 2261 G R 6184075 + 55 [bash]
> 8,0 0 14 1.943690189 2261 P N [bash]
> 8,0 0 15 1.943692075 2261 I R 6184075 + 55 [bash]
> 8,0 0 16 1.943712119 2261 D R 6184075 + 55 [bash]
> 8,0 0 17 1.943718684 2261 Q R 6184130 + 55 [bash]
> 8,0 0 18 1.943721897 2261 G R 6184130 + 55 [bash]
> 8,0 0 19 1.943726576 2261 P N [bash]
> 8,0 0 20 1.943727763 2261 I R 6184130 + 55 [bash]
> 8,0 0 21 1.943731675 2261 Q R 6184241 + 56 [bash]
> 8,0 0 22 1.943735167 2261 G R 6184241 + 56 [bash]
> 8,0 0 23 1.943739497 2261 I R 6184241 + 56 [bash]
> 8,0 0 24 1.943742081 2261 Q R 6184185 + 56 [bash]
> 8,0 0 25 1.943744875 2261 M R 6184185 + 56 [bash]
> 8,0 0 26 1.943753535 2261 Q R 6184352 + 55 [bash]
> 8,0 0 27 1.943756538 2261 G R 6184352 + 55 [bash]
> 8,0 0 28 1.943760868 2261 I R 6184352 + 55 [bash]
> 8,0 0 29 1.943763522 2261 Q R 6184407 + 55 [bash]
> 8,0 0 30 1.943766316 2261 M R 6184407 + 55 [bash]
> 8,0 0 31 1.943770506 2261 Q R 6184297 + 55 [bash]
> 8,0 0 32 1.943772951 2261 F R 6184297 + 55 [bash]
> 8,0 0 33 1.943780843 2261 Q R 6184462 + 55 [bash]
> 8,0 0 34 1.943783776 2261 M R 6184462 + 55 [bash]
> 8,0 0 35 1.944444684 0 UT N [swapper] 2
> 8,0 0 36 1.944453624 10 U N [kblockd/0] 2
> 8,0 0 37 1.944470595 10 D R 6184130 + 387 [kblockd/0]
> 8,0 0 38 1.970340853 0 C R 6184075 + 55 [0]
> 8,0 0 39 1.973576739 0 C R 6184130 + 387 [0]
>
>
Oops, that was incomplete information to reproduce the issue.

typedef struct {
int IoSize;
char *Mem;
} DB_MEM_LIST, *pDB_MEM_LIST;

DB_MEM_LIST dbMemList[NUM_BIO] = {
{ 0x6e00, NULL },
{ 0x6e00, NULL },
{ 0x7000, NULL },
{ 0x7000, NULL },
{ 0x6e00, NULL },
{ 0x6e00, NULL },
{ 0x6e00, NULL },
{ 0x6e00, NULL },
};


// Allocate Memory.
char *Mem0 = kmalloc(dbMemList[0].IoSize, GFP_KERNEL);
char *Mem1 = kmalloc(dbMemList[1].IoSize, GFP_KERNEL);
char *Mem2 = kmalloc((dbMemList[2].IoSize + dbMemList[3].IoSize +
dbMemList[4].IoSize), GFP_KERNEL);
char *Mem5 = kmalloc(dbMemList[5].IoSize, GFP_KERNEL);
char *Mem6 = kmalloc(dbMemList[6].IoSize, GFP_KERNEL);
char *Mem7 = kmalloc(dbMemList[7].IoSize, GFP_KERNEL);

dbMemList[0].Mem = Mem0;
dbMemList[1].Mem = Mem1;
dbMemList[2].Mem = Mem2;
dbMemList[3].Mem = dbMemList[2].Mem + dbMemList[2].IoSize;
dbMemList[4].Mem = dbMemList[3].Mem + dbMemList[3].IoSize;
dbMemList[5].Mem = Mem5;
dbMemList[6].Mem = Mem6;
dbMemList[7].Mem = Mem7;


int Loop;
struct bio *BioList[NUM_BIO] = { NULL };
struct bio *Bio = NULL;
long Sector;


/*
** Allocate BIOs.
*/
for(Loop = 0; Loop < NUM_BIO; Loop++)
BioList[Loop] = bio_alloc(GFP_KERNEL, NUMBER_OF_VECS);

for(Loop = 0; Loop < NUM_BIO; Loop++) {
Bio = BioList[Loop];
Bio->bi_sector = Sector;
Bio->bi_bdev = Bdev;
Bio->bi_end_io = db_end_io;
if (Loop == 0) {
if (!bio_add_page(BioList[Loop],
virt_to_page(dbMemList[Loop].Mem),
0x3000,
offset_in_page(dbMemList[Loop].Mem))) {
printk("NIK: bio add page failed- %dA\n", Loop);
}
if (!bio_add_page(BioList[Loop],
virt_to_page(dbMemList[7].Mem),
0x3e00,
offset_in_page(dbMemList[7].Mem))) {
printk("NIK: bio add page failed- %dB\n", Loop);
}
} else if (Loop == 7) {
if (!bio_add_page(BioList[Loop],
virt_to_page(dbMemList[Loop].Mem),
0x2000,
offset_in_page(dbMemList[Loop].Mem))) {
printk("NIK: bio add page failed- %dA\n", Loop);
}
if (!bio_add_page(BioList[Loop],
virt_to_page(dbMemList[7].Mem),
0x4e00,
offset_in_page(dbMemList[7].Mem))) {
printk("NIK: bio add page failed- %dB\n", Loop);
}
} else if (!bio_add_page(BioList[Loop],
virt_to_page(dbMemList[Loop].Mem),
dbMemList[Loop].IoSize,
offset_in_page(dbMemList[Loop].Mem))) {
printk("NIK: bio add page failed %d\n", Loop);
}
Sector += (Bio->bi_size / 512);
}

submit_bio(READ, BioList[0]); // 1
submit_bio(READ, BioList[1]); // 2
submit_bio(READ, BioList[3]); // 4
submit_bio(READ, BioList[2]); // 3
submit_bio(READ, BioList[5]); // 6
submit_bio(READ, BioList[6]); // 7
submit_bio(READ, BioList[4]); // 5
submit_bio(READ, BioList[7]); // 8

Thanks
Nikanth Karthikesan
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