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 9:27 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Thu, Jan 26, 2023 at 08:10:26AM -0800, Suren Baghdasaryan wrote:
> > 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;
> > }
> >
>
> Offhand, I'm not thinking of a case where that really matters and as they
> are not necessarily ordered, it's raising a read flag so yes, it definitely
> it needs a comment if the ordering matters.
>
> > 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.
>
> Ok, that seems fair but still needs a comment on why a mod_vm_flags is
> not necessarily equivalent to a set_vm_flags + clear_vm_flags in terms of
> correctness if that is indeed the case.
>
> > 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?
> >
>
> My concerns are entirely with the callers, not the implementation. If
> someone is modifying a call site using mod_vm_flags, they have to read
> through all the preceding logic to ensure the final combination of flags
> is valid.  It's a code maintenance issue, not a correctness issue.

Got it. I'll modify the implementation to make a single assignment and
will add a comment to use only when order doesn't matter.
Thanks!

>
> --
> Mel Gorman
> SUSE Labs