linux-kernel - Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

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Date:   Sat, 7 Aug 2021 03:02:33 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the
 proper place

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.