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

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Message-ID: <CAJuCfpHP6hQAWZr2exZEXOzLbMNU_c9qNNc7pa2NYAhYLe=EKQ@mail.gmail.com>
Date:   Thu, 26 Jan 2023 08:18:31 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
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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 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.

>
> > Introduce mod_vm_flags_nolock to be used in such situation, when VMA is
> > not part of VMA tree and locking it is not required.
>
> Instead of such situations, describe in as "used when the caller takes
> responsibility for the required locking".

Ack.

>
> > 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.
Thanks!

>
> --
> Mel Gorman
> SUSE Labs