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Subject[PATCH 0/3] use div64_ul() instead of div_u64() if the divisor is unsigned long
We were first inspired by commit b0ab99e7736a
("sched: Fix possible divide by zero in avg_atom () calculation"),
then refer to the recently analyzed mm code,
we found this suspicious place.
201 if (min) {
202 min *= this_bw;
203 do_div(min, tot_bw);
204 }

And we also disassembled and confirmed it:
/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 201
0xffffffff811c37da <__wb_calc_thresh+234>: xor %r10d,%r10d
0xffffffff811c37dd <__wb_calc_thresh+237>: test %rax,%rax
0xffffffff811c37e0 <__wb_calc_thresh+240>: je 0xffffffff811c3800 <__wb_calc_thresh+272>
/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 202
0xffffffff811c37e2 <__wb_calc_thresh+242>: imul %r8,%rax
/usr/src/debug/kernel-4.9.168-016.ali3000/linux-4.9.168-016.ali3000.alios7.x86_64/mm/page-writeback.c: 203
0xffffffff811c37e6 <__wb_calc_thresh+246>: mov %r9d,%r10d ---> truncates it to 32 bits here
0xffffffff811c37e9 <__wb_calc_thresh+249>: xor %edx,%edx
0xffffffff811c37eb <__wb_calc_thresh+251>: div %r10
0xffffffff811c37ee <__wb_calc_thresh+254>: imul %rbx,%rax
0xffffffff811c37f2 <__wb_calc_thresh+258>: shr $0x2,%rax
0xffffffff811c37f6 <__wb_calc_thresh+262>: mul %rcx
0xffffffff811c37f9 <__wb_calc_thresh+265>: shr $0x2,%rdx
0xffffffff811c37fd <__wb_calc_thresh+269>: mov %rdx,%r10

This series use div64_ul() instead of div_u64() if the divisor
is unsigned long, to avoid truncation to 32-bit on 64-bit platforms.

Cc: Andrew Morton <>
Cc: Qian Cai <>
Cc: Tejun Heo <>
Cc: Jens Axboe <>

Wen Yang (3):
mm/page-writeback.c: avoid potential division by zero in
mm/page-writeback.c: use div64_ul() for u64-by-unsigned-long divide
mm/page-writeback.c: improve arithmetic divisions

mm/page-writeback.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)


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