• David Rientjes's avatar
    mm, pcp: allow restoring percpu_pagelist_fraction default · 7cd2b0a3
    David Rientjes authored
    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:
    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.
    Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
    Reported-by: default avatarOleg Drokin <green@linuxhacker.ru>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
sysctl.c 62.6 KB