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From: Peter Xu <peterx@redhat.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	rcampbell@nvidia.com, linux-doc@vger.kernel.org,
	nouveau@lists.freedesktop.org, hughd@google.com,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	hch@infradead.org, bskeggs@redhat.com, jgg@nvidia.com,
	shakeelb@google.com, jhubbard@nvidia.com, willy@infradead.org,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v10 07/10] mm: Device exclusive memory access
Date: Thu, 10 Jun 2021 14:04:35 -0400	[thread overview]
Message-ID: <YMJUM4sx1VclR7oz@t490s> (raw)
In-Reply-To: <2773835.D95cIkl9rl@nvdebian>

On Thu, Jun 10, 2021 at 10:18:25AM +1000, Alistair Popple wrote:
> > > The main problem is split_huge_pmd_address() unconditionally calls a mmu
> > > notifier so I would need to plumb in passing an owner everywhere which could
> > > get messy.
> > 
> > Could I ask why?  split_huge_pmd_address() will notify with CLEAR, so I'm a bit
> > confused why we need to pass over the owner.
> 
> Sure, it is the same reason we need to pass it for the exclusive notifier.
> Any invalidation during the make exclusive operation will break the mmu read
> side critical section forcing a retry of the operation. The owner field is what
> is used to filter out invalidations (such as the exclusive invalidation) that
> don't need to be retried.

Do you mean the mmu_interval_read_begin|retry() calls?

Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit
call split_huge_pmd_address(), afaict.  Since both of them use __split_huge_pmd()
internally which will generate that unwanted CLEAR notify.

If that's the case, I think it fails because split_huge_pmd_address() will
trigger that CLEAR notify unconditionally (even if it's not a thp; not sure
whether it should be optimized to not notify at all... definitely another
story), while FOLL_SPLIT_PMD will skip the notify as it calls split_huge_pmd()
instead, who checks the pmd before calling __split_huge_pmd().

Does it also mean that if there's a real THP it won't really work?  As then
FOLL_SPLIT_PMD will start to trigger that CLEAR notify too, I think..

-- 
Peter Xu


  reply	other threads:[~2021-06-10 18:04 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-07  7:58 [PATCH v10 00/10] Add support for SVM atomics in Nouveau Alistair Popple
2021-06-07  7:58 ` [PATCH v10 01/10] mm: Remove special swap entry functions Alistair Popple
2021-06-07  7:58 ` [PATCH v10 02/10] mm/swapops: Rework swap entry manipulation code Alistair Popple
2021-06-07  7:58 ` [PATCH v10 03/10] mm/rmap: Split try_to_munlock from try_to_unmap Alistair Popple
2021-06-07  7:58 ` [PATCH v10 04/10] mm/rmap: Split migration into its own function Alistair Popple
2021-06-07  7:58 ` [PATCH v10 05/10] mm: Rename migrate_pgmap_owner Alistair Popple
2021-06-08 15:16   ` Peter Xu
2021-06-07  7:58 ` [PATCH v10 06/10] mm/memory.c: Allow different return codes for copy_nonpresent_pte() Alistair Popple
2021-06-08 15:19   ` Peter Xu
2021-06-07  7:58 ` [PATCH v10 07/10] mm: Device exclusive memory access Alistair Popple
2021-06-08 18:33   ` Peter Xu
2021-06-09  9:38     ` Alistair Popple
2021-06-09 16:05       ` Peter Xu
2021-06-10  0:18         ` Alistair Popple
2021-06-10 18:04           ` Peter Xu [this message]
2021-06-10 14:21             ` Alistair Popple
2021-06-10 23:04               ` Peter Xu
2021-06-10 23:17                 ` Alistair Popple
2021-06-11  1:00                   ` Peter Xu
2021-06-11  3:43                     ` Alistair Popple
2021-06-11 15:01                       ` Peter Xu
2021-06-15  3:08                         ` Alistair Popple
2021-06-15 16:25                           ` Peter Xu
2021-06-16  2:47                             ` Alistair Popple
2021-06-07  7:58 ` [PATCH v10 08/10] mm: Selftests for exclusive device memory Alistair Popple
2021-06-07  7:58 ` [PATCH v10 09/10] nouveau/svm: Refactor nouveau_range_fault Alistair Popple
2021-06-07  7:58 ` [PATCH v10 10/10] nouveau/svm: Implement atomic SVM access Alistair Popple

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