On Mon, 26 Mar 2012, Kautuk Consul wrote:
> find_vma is called from kernel code where it is absolutely
> sure that the mm_struct arg being passed to it is non-NULL.
>
> Convert the if check to a BUG_ON.
> This will also serve the purpose of mandating that the execution
> context(user-mode/kernel-mode) be known before find_vma is called.
>
> Also fixed 2 checkpatch.pl errors in this function for the rb_node
> and vma_tmp local variables.
>
> I have tested this patch on my x86 PC and there are no BUG_ON crashes
> due to this in the course of normal desktop execution.
>
> Signed-off-by: Kautuk Consul <consul.kautuk@xxxxxxxxx>
That seems very reasonable: perhaps there was a reason for checking
find_vma()'s mm way back in the distant past, but I cannot see it now.
But please make two small changes noted below before resubmitting.
Since we ask for mm->mmap_sem to be held before calling find_vma(),
it's hard to reach here with NULL mm. There are a few strange places
in arch and drivers/media/video which appear to be taking risks by
not holding mmap_sem, but only one of them looks like it _might_ be
endangered by your change.
Ralf, that octeon_flush_cache_sigtramp() in arch/mips/mm/c-octeon.c:
is there ever a danger that it can be called with NULL current->mm? Is
current->mm set to &init_mm in the initial call from octeon_cache_init()?
> ---
> mm/mmap.c | 52 ++++++++++++++++++++++++++--------------------------
> 1 files changed, 26 insertions(+), 26 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index a7bf6a3..7589965 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1589,33 +1589,33 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
> {
> struct vm_area_struct *vma = NULL;
Please remove the " = NULL": vma is immediately set to mm->mmap_cache.
>
> - if (mm) {
> - /* Check the cache first. */
> - /* (Cache hit rate is typically around 35%.) */
> - vma = mm->mmap_cache;
> - if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
> - struct rb_node * rb_node;
> -
> - rb_node = mm->mm_rb.rb_node;
> - vma = NULL;
> -
> - while (rb_node) {
> - struct vm_area_struct * vma_tmp;
> -
> - vma_tmp = rb_entry(rb_node,
> - struct vm_area_struct, vm_rb);
> -
> - if (vma_tmp->vm_end > addr) {
> - vma = vma_tmp;
> - if (vma_tmp->vm_start <= addr)
> - break;
> - rb_node = rb_node->rb_left;
> - } else
> - rb_node = rb_node->rb_right;
> - }
> - if (vma)
> - mm->mmap_cache = vma;
> + BUG_ON(!mm);
And please remove the BUG_ON(!mm): it's a waste of space and time,
it gives very little value over the easily recognizable oops we
shall get from "vma = mm->mmap_cache" with NULL mm.
Thanks,
Hugh
> +
> + /* Check the cache first. */
> + /* (Cache hit rate is typically around 35%.) */
> + vma = mm->mmap_cache;
> + if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
> + struct rb_node *rb_node;
> +
> + rb_node = mm->mm_rb.rb_node;
> + vma = NULL;
> +
> + while (rb_node) {
> + struct vm_area_struct *vma_tmp;
> +
> + vma_tmp = rb_entry(rb_node,
> + struct vm_area_struct, vm_rb);
> +
> + if (vma_tmp->vm_end > addr) {
> + vma = vma_tmp;
> + if (vma_tmp->vm_start <= addr)
> + break;
> + rb_node = rb_node->rb_left;
> + } else
> + rb_node = rb_node->rb_right;
> }
> + if (vma)
> + mm->mmap_cache = vma;
> }
> return vma;
> }
> --
> 1.7.5.4
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