linux-kernel - Re: [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap

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Date:   Wed, 15 Sep 2021 00:01:24 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Liam Howlett <liam.howlett@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags

On Wed, Sep 15, 2021 at 01:21:30PM +1000, Alistair Popple wrote:
> On Wednesday, 15 September 2021 12:52:48 PM AEST Peter Xu wrote:
> 
> > > > The flag introduced in this patch will be a preparation for more bits defined
> > > > in the future, e.g., for a new bit in flag to show whether to persist the
> > > > upcoming uffd-wp bit in pgtable entries.
> > > 
> > > That's kind of the problem. The patch itself looks correct to me however as
> > > mentioned it is mostly reverting a previous cleanup and it's hard to tell why
> > > that's justified without the subsequent patches. Perhaps it makes the usage of
> > > zap_details a bit clearer, but a comment also would with less code.
> > > 
> > > I know you want to try and shrink the uffd-wp series but I think this patch
> > > might be easier to review if it was included as part of that series.
> > 
> > I posted it because I think it's suitable to have it even without uffd-wp.
> > 
> > I tried to explain it above on two things this patch wanted to fix:
> > 
> > Firstly the comment is wrong; we've moved back and forth on changing the
> > zap_details flags but the comment is not changing along the way and it's not
> > matching the code right now.
> > 
> > Secondly I do think we should have a flag showing explicit willingness to skip
> > swap entries.  Yes, uffd-wp is the planned new one, but my point is anyone who
> > will introduce a new user of zap_details pointer could overlook this fact.  The
> > new flag helps us to make sure someone will at least read the flags and know
> > what'll happen with it.
> > 
> > For the 2nd reasoning, I also explicitly CCed Kirill too, so Kirill can provide
> > any comment if he disagrees.  For now, I still think we should keep having such
> > a flag otherwise it could be error-prone.
> > 
> > Could you buy-in above reasoning?
> 
> Kind of, I do think it makes the usage of details a bit clearer or at least
> harder to miss. It is just that if that was the sole aim of this patch I think
> there might be simpler (less code) ways of doing so.

Yes you're right, we can add a big enough comment above zap_details to state
that, but then it'll be reverted when adding the uffd-wp flag in the other
series, because uffd-wp will still needs a way to specify !SKIP_SWAP and
KEEP_UFFD_WP.  Then it'll make the "series split" make less sense as you said.

I split the series only because I hope it could ease the reviewers, and also
that's probably the only thing I can do now to still try to smooth the process
of having a complete uffd-wp finally got proper reviewed and merged.

> 
> > Basically above is what I wanted to express in my commit message.  I hope that
> > can justify that this patch (even if extremly simple) can still be considered
> > as acceptable upstream even without uffd-wp series.
> > 
> > If you still insist on this patch not suitable for standalone merging and
> > especially if some other reviewer would think the same, I can move it back to
> > uffd-wp series for sure.  Then I'll repost this series with 4 patches only.
> 
> I won't insist, the code looks correct and it doesn't make things any less
> clear so you can put my Reviewed-by on it and perhaps leave it to Andrew or
> another reviewer to determine if this should be taken in this series or as part
> of a future uffd-wp series.

Will do; thanks.

-- 
Peter Xu