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From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place
Date: Sun, 8 Aug 2021 10:55:30 +0800	[thread overview]
Message-ID: <36956352-246a-b3c2-3ade-2a6c22e2cd5a@linux.alibaba.com> (raw)
In-Reply-To: <YQ3puWSgUvfvIYjv@casper.infradead.org>

Hi,

> On Fri, Aug 06, 2021 at 11:07:18AM +0800, Baolin Wang wrote:
>> Hi Matthew,
>>
>>> On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
>>>> We've got the expected count for anonymous page or file page by
>>>> expected_page_refs() at the beginning of migrate_page_move_mapping(),
>>>> thus we should move the page count validation a little forward to
>>>> reduce duplicated code.
>>>
>>> Please add an explanation to the changelog for why it's safe to pull
>>> this out from under the i_pages lock.
>>
>> Sure. In folio_migrate_mapping(), we are sure that the migration page was
>> isolated from lru list and locked, so I think there are no race to get the
>> page count without i_pages lock. Please correct me if I missed something
>> else. Thanks.
> 
> Unless the page has been removed from i_pages, this isn't a correct
> explanation.  Even if it has been removed from i_pages, unless an
> RCU grace period has passed, another CPU may still be able to inc the
> refcount on it (temporarily).  The same is true for the page tables,
> by the way; if someone is using get_user_pages_fast(), they may still
> be able to see the page.

I don't think this is an issue, cause now we've established a migration 
pte for this migration page under page lock. If the user want to get 
page by get_user_pages_fast(), it will wait for the page miggration 
finished by migration_entry_wait(). So I still think there is no need to 
check the migration page count under the i_pages lock.

  reply	other threads:[~2021-08-08  2:55 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-05 15:05 [PATCH 0/5] Some cleanup for page migration Baolin Wang
2021-08-05 15:05 ` [PATCH 1/5] mm: migrate: Move the page count validation to the proper place Baolin Wang
2021-08-05 15:17   ` Matthew Wilcox
2021-08-06  3:07     ` Baolin Wang
2021-08-07  2:02       ` Matthew Wilcox
2021-08-08  2:55         ` Baolin Wang [this message]
2021-08-08 10:26           ` Matthew Wilcox
2021-08-08 15:13             ` Baolin Wang
2021-08-08 16:01               ` Matthew Wilcox
2021-08-09  4:19                 ` Baolin Wang
2021-08-05 15:05 ` [PATCH 2/5] mm: migrate: Introduce a local variable to get the number of pages Baolin Wang
2021-08-05 17:42   ` Yang Shi
2021-08-05 15:05 ` [PATCH 3/5] mm: migrate: Fix the incorrect function name in comments Baolin Wang
2021-08-05 17:26   ` Yang Shi
2021-08-09 13:59   ` Alistair Popple
2021-08-05 15:05 ` [PATCH 4/5] mm: migrate: Change to use bool type for 'page_was_mapped' Baolin Wang
2021-08-05 17:34   ` Yang Shi
2021-08-05 15:06 ` [PATCH 5/5] mm: migrate: Remove redundant goto labels Baolin Wang
2021-08-05 19:54   ` Yang Shi
2021-08-06  3:20     ` Baolin Wang
2021-08-06 17:17       ` Yang Shi
2021-08-08  2:56         ` Baolin Wang

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