Hello!
I am very happy that someone has found this issue.
I have been suffering from rather random SIGBUS errors in similar conditions described by the author.
I don't have much troubleshooting information to provide, however, I hit the issue regularly so I could investigate during that.
How do you debug such an issue? I tried a debugger etc. but besides crashing with SIGBUS, I couldnt get any other meaningful information.
dftxbs3e
On 12/3/19 1:50 PM, Jan Stancek wrote:
Hi,
(This bug report is summary from thread [1] with some additions)
User-space binaries on Power9 ppc64le (with 64k pages) on xfs filesystem are sporadically hitting SIGBUS:
---------- 8< ---------- (gdb) r Starting program: /mnt/testarea/ltp/testcases/bin/genasin
Program received signal SIGBUS, Bus error. dl_main (phdr=0x10000040, phnum=<optimized out>, user_entry=0x7fffffffe760, auxv=<optimized out>) at rtld.c:1362 1362 switch (ph->p_type)
(gdb) p ph $1 = (const Elf64_Phdr *) 0x10000040
(gdb) p *ph Cannot access memory at address 0x10000040
(gdb) info proc map process 1110670 Mapped address spaces:
Start Addr End Addr Size Offset objfile 0x10000000 0x10010000 0x10000 0x0 /mnt/testarea/ltp/testcases/bin/genasin 0x10010000 0x10030000 0x20000 0x0 /mnt/testarea/ltp/testcases/bin/genasin 0x7ffff7f90000 0x7ffff7fb0000 0x20000 0x0 [vdso] 0x7ffff7fb0000 0x7ffff7fe0000 0x30000 0x0 /usr/lib64/ld-2.30.so 0x7ffff7fe0000 0x7ffff8000000 0x20000 0x20000 /usr/lib64/ld-2.30.so 0x7ffffffd0000 0x800000000000 0x30000 0x0 [stack](gdb) x/1x 0x10000040 0x10000040: Cannot access memory at address 0x10000040 ---------- >8 ----------
When this happens the binary continues to hit SIGBUS until page is released, for example by: echo 3 > /proc/sys/vm/drop_caches
The issue goes back to at least v4.19.
I can semi-reliably reproduce it with LTP is installed to /mnt/testarea/ltp by: while [ True ]; do echo 3 > /proc/sys/vm/drop_caches rm -f /mnt/testarea/ltp/results/RUNTEST.log /mnt/testarea/ltp/output/RUNTEST.run.log ./runltp -p -d results -l RUNTEST.log -o RUNTEST.run.log -f math grep FAIL /mnt/testarea/ltp/results/RUNTEST.log && exit 1 done
and some stress activity in other terminal (e.g. kernel build). Sometimes in minutes, sometimes in hours. It is not reliable enough to get meaningful bisect results.
My theory is that there's a race in iomap. There appear to be interleaved calls to iomap_set_range_uptodate() for same page with varying offset and length. Each call sees bitmap as _not_ entirely "uptodate" and hence doesn't call SetPageUptodate(). Even though each bit in bitmap ends up uptodate by the time all calls finish.
For example, with following traces:
iomap_set_range_uptodate() ... if (uptodate && !PageError(page)) SetPageUptodate(page);
if (mycheck(page)) {trace_printk("page: %px, iop: %px, uptodate: %d, !PageError(page): %d, flags: %lx\n", page, iop, uptodate, !PageError(page), page->flags);trace_printk("first: %u, last: %u, off: %u, len: %u, i: %u\n", first, last, off, len, i);}I get: genacos-18471 [057] .... 162.465730: iomap_readpages: mapping: c000003f185a1ab0 genacos-18471 [057] .... 162.465732: iomap_page_create: iomap_page_create page: c00c00000fe26180, page->private: 0000000000000000, iop: c000003fc70a19c0, flags: 3ffff800000001 genacos-18471 [057] .... 162.465736: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002001 genacos-18471 [057] .... 162.465736: iomap_set_range_uptodate: first: 1, last: 14, off: 4096, len: 57344, i: 16 <idle>-0 [060] ..s. 162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081 <idle>-0 [061] ..s. 162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081 <idle>-0 [060] ..s. 162.534864: iomap_set_range_uptodate: first: 0, last: 0, off: 0, len: 4096, i: 16 <idle>-0 [061] ..s. 162.534864: iomap_set_range_uptodate: first: 15, last: 15, off: 61440, len: 4096, i: 16
This page doesn't have Uptodate flag set, which leads to filemap_fault() returning VM_FAULT_SIGBUS:
crash> p/x ((struct page *) 0xc00c00000fe26180)->flags $1 = 0x3ffff800002032
crash> kmem -g 0x3ffff800002032 FLAGS: 3ffff800002032 PAGE-FLAG BIT VALUE PG_error 1 0000002 PG_dirty 4 0000010 PG_lru 5 0000020 PG_private_2 13 0002000 PG_fscache 13 0002000 PG_savepinned 4 0000010 PG_double_map 13 0002000
But iomap_page->uptodate in page->private suggests all bits are uptodate:
crash> p/x ((struct page *) 0xc00c00000fe26180)->private $2 = 0xc000003fc70a19c0
crash> p/x ((struct iomap_page *) 0xc000003fc70a19c0)->uptodate $3 = {0xffff, 0x0}
It appears (after ~4 hours) that I can avoid the problem if I split the loop so that bits are checked only after all updates are made. Not sure if this correct approach, or just making it less reproducible:
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index e25901ae3ff4..abe37031c93d 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -131,7 +131,11 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len) for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) { if (i >= first && i <= last) set_bit(i, iop->uptodate);
else if (!test_bit(i, iop->uptodate))
}for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {if (i >= first && i <= last)continue;if (!test_bit(i, iop->uptodate)) uptodate = false; } }Thanks, Jan
[1] https://lore.kernel.org/stable/1420623640.14527843.1575289859701.JavaMail.zi...