Re: [PATCH v3 4/7] mm: replace vma->vm_flags direct modifications with modifier calls

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 26, 2023 at 7:10 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Wed, Jan 25, 2023 at 03:35:51PM -0800, Suren Baghdasaryan wrote:
> > Replace direct modifications to vma->vm_flags with calls to modifier
> > functions to be able to track flag changes and to keep vma locking
> > correctness.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
>
> Minor comments that are safe to ignore.
>
> I think a better name for mod_vm_flags is set_clear_vm_flags to hint that
> the first flags are to be set and the second flags are to be cleared.
> For this patch, it doesn't matter, but it might avoid accidental swapping
> in the future.
>
> reset_vm_flags might also be better named as reinit_vma_flags (or
> vma_flags_reinit). Maybe also encourage the use of [set|clear_mod]_vm_flags
> where possible in the comment to track exactly what is changing and
> why. Some cases like userfaultfd just want to clear __VM_UFFD_FLAGS but
> altering the flow in this patch is inappropriate and error prone. Others
> such as the infiniband changes and madvise are a lot more complex.

That's a good point, but I don't want people to use mod_vm_flags() for
the cases when the order of set/clear really matters. In such cases
set_vm_flags() and clear_vm_flags() should be explicitly used. Maybe
to make that clear I should add a comment and rewrite the functions
as:

void mod_vm_flags(vma, set, clear) {
    vma.vm_flags = vma.vm_flags | set & clear;
}

In this patchset it's not that obvious but mod_vm_flags() was really
introduced in the original per-VMA lock patchset for efficiency to
avoid taking extra per-VMA locks. A combo of
set_vm_flags()+clear_vm_flags() would try to retake the same per-VMA
lock in the second call while mod_vm_flags() takes the lock only once
and does both operations. Not a huge overhead because we check if the
lock is already taken and bail out early but still...
So, would the above modification to mod_vm_flags() address your concern?

>
> --
> Mel Gorman
> SUSE Labs