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From: Alistair Popple <apopple@nvidia.com>
To: Ralph Campbell <rcampbell@nvidia.com>
Cc: linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	kvm-ppc@vger.kernel.org, linux-mm@kvack.org, bskeggs@redhat.com,
	akpm@linux-foundation.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [Nouveau] [PATCH v4 4/8] mm/rmap: Split migration into its own function
Date: Tue, 9 Mar 2021 11:01:08 +1100
Message-ID: <3142506.6Va0CHJdLq@nvdebian> (raw)
In-Reply-To: <9fa77684-149a-e6ed-296d-dc852aecea97@nvidia.com>

On Tuesday, 9 March 2021 5:58:12 AM AEDT Ralph Campbell wrote:
> 
> On 3/3/21 10:16 PM, Alistair Popple wrote:
> > Migration is currently implemented as a mode of operation for
> > try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
> > or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.
> > 
> > However it does not have much in common with the rest of the unmap
> > functionality of try_to_unmap_one() and thus splitting it into a
> > separate function reduces the complexity of try_to_unmap_one() making it
> > more readable.
> > 
> > Several simplifications can also be made in try_to_migrate_one() based
> > on the following observations:
> > 
> >   - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
> >   - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
> >   - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.
> > 
> > TTU_SPLIT_FREEZE is a special case of migration used when splitting an
> > anonymous page. This is most easily dealt with by calling the correct
> > function from unmap_page() in mm/huge_memory.c  - either
> > try_to_migrate() for PageAnon or try_to_unmap().
> > 
> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> 
> Looks reasonable to me. I do worry a bit about code duplication.

I was initially concerned about this when splitting try_to_unmap_one() up but  
most of the code paths ended up being pretty orthogonal and I think the 
clarity gained from separating them is worth a small amount of duplication.

> At some point in the future, it might be discovered that other combinations
> of TTU_XXX flags are needed in which case a careful check of 
try_to_migrate()
> and try_to_unmap() will be needed.

I wanted to keep the code as simple as possible by removing all dead code 
paths that that weren't in use today.

I think this is likely to be more of an issue if new TTU_XXX flags are added 
as there are already explicit (and hopefully exhaustive) checks in 
try_to_migrate() and try_to_unmap() for unsupported combinations of existing 
flags. To avoid ending up in the same situation again I would rather not have 
more TTU_XXX flags added if at all possible though.

> Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

Thanks.

 - Alistair



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Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-04  6:16 [Nouveau] [PATCH v4 0/8] Add support for SVM atomics in Nouveau Alistair Popple
2021-03-04  6:16 ` [Nouveau] [PATCH v4 1/8] mm: Remove special swap entry functions Alistair Popple
2021-03-08 18:24   ` Ralph Campbell
2021-03-04  6:16 ` [Nouveau] [PATCH v4 2/8] mm/swapops: Rework swap entry manipulation code Alistair Popple
2021-03-08 18:30   ` Ralph Campbell
2021-03-04  6:16 ` [Nouveau] [PATCH v4 3/8] mm/rmap: Split try_to_munlock from try_to_unmap Alistair Popple
2021-03-08 18:39   ` Ralph Campbell
2021-03-04  6:16 ` [Nouveau] [PATCH v4 4/8] mm/rmap: Split migration into its own function Alistair Popple
2021-03-08 18:58   ` Ralph Campbell
2021-03-09  0:01     ` Alistair Popple [this message]
2021-03-04  6:16 ` [Nouveau] [PATCH v4 5/8] mm: Device exclusive memory access Alistair Popple
2021-03-08 19:44   ` Ralph Campbell
2021-03-09  1:17     ` Alistair Popple
2021-03-04  6:16 ` [Nouveau] [PATCH v4 6/8] mm: Selftests for exclusive device memory Alistair Popple
2021-03-04  6:16 ` [Nouveau] [PATCH v4 7/8] nouveau/svm: Refactor nouveau_range_fault Alistair Popple
2021-03-04  6:16 ` [Nouveau] [PATCH v4 8/8] nouveau/svm: Implement atomic SVM access Alistair Popple

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