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From: Peter Xu <peterx@redhat.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	linux-mm@kvack.org, Miaohe Lin <linmiaohe@huawei.com>,
	David Hildenbrand <david@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Yang Shi <shy828301@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Jerome Glisse <jglisse@redhat.com>,
	Liam Howlett <liam.howlett@oracle.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags
Date: Wed, 15 Sep 2021 00:01:24 -0400	[thread overview]
Message-ID: <YUFwFGB2JElaBOGt@xz-m1.local> (raw)
In-Reply-To: <2497776.C4p5gPNQJS@nvdebian>

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


  reply	other threads:[~2021-09-15  4:01 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 16:35 [PATCH v3 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu
2021-09-08 16:35 ` [PATCH v3 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte Peter Xu
2021-09-08 16:36 ` [PATCH v3 2/5] mm: Clear vmf->pte after pte_unmap_same() returns Peter Xu
2021-09-08 16:36 ` [PATCH v3 3/5] mm: Drop first_index/last_index in zap_details Peter Xu
2021-09-09  2:54   ` Liam Howlett
2021-09-09 18:13     ` Peter Xu
2021-09-08 16:36 ` [PATCH v3 4/5] mm: Add zap_skip_check_mapping() helper Peter Xu
2021-09-09  1:16   ` Alistair Popple
2021-09-08 16:36 ` [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags Peter Xu
2021-09-15  2:25   ` Alistair Popple
2021-09-15  2:52     ` Peter Xu
2021-09-15  3:21       ` Alistair Popple
2021-09-15  4:01         ` Peter Xu [this message]
2021-09-14 16:37 ` [PATCH v3 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu

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