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    SubjectRe: [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user_ram
    ----- On Oct 16, 2018, at 2:30 PM, rostedt rostedt@goodmis.org wrote:

    > On Wed, 10 Oct 2018 15:19:24 -0400
    > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
    >
    >> + * vm_unmap_user_ram - unmap linear kernel address space set up by
    >> vm_map_user_ram
    >> + * @mem: the pointer returned by vm_map_user_ram
    >> + * @count: the count passed to that vm_map_user_ram call (cannot unmap partial)
    >> + */
    >> +void vm_unmap_user_ram(const void *mem, unsigned int count)
    >> +{
    >> + unsigned long size = (unsigned long)count << PAGE_SHIFT;
    >> + unsigned long addr = (unsigned long)mem;
    >> + struct vmap_area *va;
    >> +
    >> + might_sleep();
    >> + BUG_ON(!addr);
    >> + BUG_ON(addr < VMALLOC_START);
    >> + BUG_ON(addr > VMALLOC_END);
    >> + BUG_ON(!PAGE_ALIGNED(addr));
    >> +
    >> + debug_check_no_locks_freed(mem, size);
    >> + va = find_vmap_area(addr);
    >> + BUG_ON(!va);
    >> + free_unmap_vmap_area(va);
    >> +}
    >> +EXPORT_SYMBOL(vm_unmap_user_ram);
    >> +
    >
    > Noticing this from Sergey's question in another patch, why are you
    > using BUG_ON()? That's rather extreme and something we are trying to
    > avoid adding more of (I still need to remove the BUG_ON()s I've added
    > over ten years ago). I don't see why all these BUG_ON's can't be turned
    > into:
    >
    > if (WARN_ON(x))
    > return;

    I borrowed the code from vm_unmap_ram(), which has the following checks:

    BUG_ON(!addr);
    BUG_ON(addr < VMALLOC_START);
    BUG_ON(addr > VMALLOC_END);
    BUG_ON(!PAGE_ALIGNED(addr));
    [...]
    va = find_vmap_area(addr);
    BUG_ON(!va);

    The expectation here is that inputs to vm_unmap_ram() should always come from
    vm_map_ram(), so an erroneous input is an internal kernel bug. I applied the
    same logic to vm_unmap_user_ram() and vm_map_user_ram().

    Should we turn all those BUG_ON() into if (WARN_ON(x)) return; in vm_{map,unmap}_ram
    as well ?

    Thanks,

    Mathieu


    >
    > -- Steve

    --
    Mathieu Desnoyers
    EfficiOS Inc.
    http://www.efficios.com

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