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From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>,
	linux-mm@kvack.org
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Yang Shi <shy828301@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Jerome Glisse <jglisse@redhat.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Alistair Popple <apopple@nvidia.com>,
	Axel Rasmussen <axelrasmussen@google.com>
Subject: Re: [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte
Date: Fri, 3 Sep 2021 09:42:34 +0200	[thread overview]
Message-ID: <2f1bfe82-9bb7-957c-2b32-2ccf8a48e70a@redhat.com> (raw)
In-Reply-To: <20210902201721.52796-2-peterx@redhat.com>

On 02.09.21 22:17, Peter Xu wrote:
> It was conditionally done previously, as there's one shmem special case that we
> use SetPageDirty() instead.  However that's not necessary and it should be
> easier and cleaner to do it unconditionally in mfill_atomic_install_pte().
> 
> The most recent discussion about this is here, where Hugh explained the history
> of SetPageDirty() and why it's possible that it's not required at all:
> 
> https://lore.kernel.org/lkml/alpine.LSU.2.11.2104121657050.1097@eggly.anvils/
> 
> Currently mfill_atomic_install_pte() has three callers:
> 
>          1. shmem_mfill_atomic_pte
>          2. mcopy_atomic_pte
>          3. mcontinue_atomic_pte
> 
> After the change: case (1) should have its SetPageDirty replaced by the dirty
> bit on pte (so we unify them together, finally), case (2) should have no
> functional change at all as it has page_in_cache==false, case (3) may add a
> dirty bit to the pte.  However since case (3) is UFFDIO_CONTINUE for shmem,
> it's merely 100% sure the page is dirty after all, so should not make a real
> difference either.

Would it be worth adding VM_BUG_ON() to make sure that "100%" is really 
the case?

> 
> This should make it much easier to follow on which case will set dirty for
> uffd, as we'll simply set it all now for all uffd related ioctls.  Meanwhile,
> no special handling of SetPageDirty() if there's no need.

To me this all sounds sane, but I'm certainly not an expert on that 
code, so ...

-- 
Thanks,

David / dhildenb


  reply	other threads:[~2021-09-03  7:42 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-02 20:17 [PATCH v2 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu
2021-09-02 20:17 ` [PATCH v2 1/5] mm/shmem: Unconditionally set pte dirty in mfill_atomic_install_pte Peter Xu
2021-09-03  7:42   ` David Hildenbrand [this message]
2021-09-03 20:00     ` Peter Xu
2021-09-03 20:02       ` David Hildenbrand
2021-09-02 20:17 ` [PATCH v2 2/5] mm: Clear vmf->pte after pte_unmap_same() returns Peter Xu
2021-09-08  1:12   ` Liam Howlett
2021-09-02 20:17 ` [PATCH v2 3/5] mm: Drop first_index/last_index in zap_details Peter Xu
2021-09-02 20:18 ` [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper Peter Xu
2021-09-03  0:58   ` Alistair Popple
2021-09-03  1:39     ` Peter Xu
2021-09-03  1:50       ` Alistair Popple
2021-09-03  7:07         ` David Hildenbrand
2021-09-02 20:18 ` [PATCH v2 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags Peter Xu
2021-09-03  7:25   ` David Hildenbrand
2021-09-03  7:31     ` David Hildenbrand
2021-09-08  0:43     ` Peter Xu
2021-09-08  0:35 ` [PATCH v2 0/5] mm: A few cleanup patches around zap, shmem and uffd Peter Xu

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