Issue in count_mm_mlocked_page_nr() with commit afcf76df48f9 ("mm/mlock: use vma iterator and maple state instead of vma linked list")

[Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx>]

Dear Matthew, dear Liam,

The commit afcf76df48f9 ("mm/mlock: use vma iterator and maple state instead
of vma linked list") refactors the function count_mm_mlocked_page_nr() in
mm/mlock.c, such that the special case:

        if (mm == NULL)
                mm = current->mm;

is now dead code. Before the refactoring, after this if statement, the variable

mm was used further in vma = find_vma(mm, start) in the following line, but now,
with the introduction of VMA_ITERATOR(vmi, mm, start) before the if statement,
the assignment within the if statement is simply ignored.

I assume that:
- If the semantics of this function shall be preserved, the VMA_ITERATOR macro
needs to move after the if statement.

- or, if the consideration of the special case on mm == NULL is actually
obsolete with this refactoring, it could be simply deleted.

I will keep it up to you how this issue is correctly resolved.

This was discovered due to tracking all dead stores that clang-analyzer reports
on an x86 defconfig. The underlying issue here is more severe than "just a dead
store in the kernel code base", but it is probably easy for you to resolve.

Best regards,

Lukas