Oleg reports a division by zero error on zero-length write() to the percpu_pagelist_fraction sysctl: divide error: 0000 [#1] SMP DEBUG_PAGEALLOC CPU: 1 PID: 9142 Comm: badarea_io Not tainted 3.15.0-rc2-vm-nfs+ #19 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 task: ffff8800d5aeb6e0 ti: ffff8800d87a2000 task.ti: ffff8800d87a2000 RIP: 0010: percpu_pagelist_fraction_sysctl_handler+0x84/0x120 RSP: 0018:ffff8800d87a3e78 EFLAGS: 00010246 RAX: 0000000000000f89 RBX: ffff88011f7fd000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000010 RBP: ffff8800d87a3e98 R08: ffffffff81d002c8 R09: ffff8800d87a3f50 R10: 000000000000000b R11: 0000000000000246 R12: 0000000000000060 R13: ffffffff81c3c3e0 R14: ffffffff81cfddf8 R15: ffff8801193b0800 FS: 00007f614f1e9740(0000) GS:ffff88011f440000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b CR2: 00007f614f1fa000 CR3: 00000000d9291000 CR4: 00000000000006e0 Call Trace: proc_sys_call_handler+0xb3/0xc0 proc_sys_write+0x14/0x20 vfs_write+0xba/0x1e0 SyS_write+0x46/0xb0 tracesys+0xe1/0xe6 However, if the percpu_pagelist_fraction sysctl is set by the user, it is also impossible to restore it to the kernel default since the user cannot write 0 to the sysctl. This patch allows the user to write 0 to restore the default behavior. It still requires a fraction equal to or larger than 8, however, as stated by the documentation for sanity. If a value in the range [1, 7] is written, the sysctl will return EINVAL. This successfully solves the divide by zero issue at the same time.
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| @@ -136,7 +136,6 @@ static unsigned long dirty_bytes_min = 2 * PAGE_SIZE; · 135 unmodified lines | |||
| 136 | /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */ | 136 | /* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */ |
| 137 | static int maxolduid = 65535; | 137 | static int maxolduid = 65535; |
| 138 | static int minolduid; | 138 | static int minolduid; |
| 139 | static int min_percpu_pagelist_fract = 8; | ||
| 140 | 139 | ||
| 141 | static int ngroups_max = NGROUPS_MAX; | 140 | static int ngroups_max = NGROUPS_MAX; |
| 142 | static const int cap_last_cap = CAP_LAST_CAP; | 141 | static const int cap_last_cap = CAP_LAST_CAP; |
| @@ -1317,7 +1316,7 @@ static struct ctl_table vm_table[] = { · 1174 unmodified lines | |||
| 1317 | .maxlen = sizeof(percpu_pagelist_fraction), | 1316 | .maxlen = sizeof(percpu_pagelist_fraction), |
| 1318 | .mode = 0644, | 1317 | .mode = 0644, |
| 1319 | .proc_handler = percpu_pagelist_fraction_sysctl_handler, | 1318 | .proc_handler = percpu_pagelist_fraction_sysctl_handler, |
| 1320 | .extra1 = &min_percpu_pagelist_fract, | 1319 | .extra1 = &zero, |
| 1321 | }, | 1320 | }, |
| 1322 | #ifdef CONFIG_MMU | 1321 | #ifdef CONFIG_MMU |
| 1323 | { | 1322 | { |