linux-kernel - Re: [PATCH v3 6/7] mm: introduce mod_vm_flags_nolock and use it in untrack

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Date:   Thu, 26 Jan 2023 09:34:35 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
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Subject: Re: [PATCH v3 6/7] mm: introduce mod_vm_flags_nolock and use it in untrack_pfn

On Thu, Jan 26, 2023 at 9:32 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Thu, Jan 26, 2023 at 08:18:31AM -0800, Suren Baghdasaryan wrote:
> > On Thu, Jan 26, 2023 at 7:47 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > On Wed, Jan 25, 2023 at 03:35:53PM -0800, Suren Baghdasaryan wrote:
> > > > In cases when VMA flags are modified after VMA was isolated and mmap_lock
> > > > was downgraded, flags modifications would result in an assertion because
> > > > mmap write lock is not held.
> > >
> > > Add note that it's also used during exit when the locking of the VMAs
> > > becomes irrelevant (mm users is 0, should be no VMA modifications taking
> > > place other than zap).
> >
> > Ack.
> >
> > >
> > > The typical naming pattern when a caller either knows it holds the necessary
> > > lock or knows it does not matter is __mod_vm_flags()
> >
> > Ok. It sounds less explicit but plenty of examples, so I'm fine with
> > such rename. Will apply in the next version.
> >
>
> It might be a personal thing. nolock to me is ambigious because it might
> mean "lock is already held", "no lock is necessary" or "no lock is acquired"
> where as *for me*, calling foo vs __foo *usually* means "direct callers of
> __foo take care of the locking, memory ordering, per-cpu pinning details etc"
> depending on the context. Of course, this convention is not universally true.
>
> > > > Pass a hint to untrack_pfn to conditionally use mod_vm_flags_nolock for
> > > > flags modification and to avoid assertion.
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > >
> > > Patch itself looks ok. It strays close to being "conditional locking"
> > > though which might attract some complaints.
> >
> > The description seems to accurately describe what's done here but I'm
> > open to better suggestions.
>
> I don't have alternative suggestions but if someone else reads the patch and
> says "this is conditional locking", you can at least claim that someone
> else considered "conditional locking" and didn't think it was a major
> problem in this specific patch.

Perfect. Thanks!

>
> --
> Mel Gorman
> SUSE Labs