Re: [PATCH] perf: rework memory accounting in perf_mmap()

[Peter Zijlstra]

On Wed, Sep 04, 2019 at 02:46:18PM -0700, Song Liu wrote:
> perf_mmap() always increases user->locked_vm. As a result, "extra" could
> grow bigger than "user_extra", which doesn't make sense. Here is an
> example case:
> 
> Note: Assume "user_lock_limit" is very small.
> | # of perf_mmap calls |vma->vm_mm->pinned_vm|user->locked_vm|
> | 0                    | 0                   | 0             |
> | 1                    | user_extra          | user_extra    |
> | 2                    | 3 * user_extra      | 2 * user_extra|
> | 3                    | 6 * user_extra      | 3 * user_extra|
> | 4                    | 10 * user_extra     | 4 * user_extra|
> 
> Fix this by maintaining proper user_extra and extra.

Aah, indeed.

Also, this code is unreadable (which is mostly my own fault I suppose)
:/