Re: [rfc][patch] remove racy sync_page?

Nick Piggin
2006-05-31T15:22:19Z

Hugh Dickins wrote:
> On Wed, 31 May 2006, Nick Piggin wrote:
>
>>Jens Axboe wrote:
>>
>>>Maybe I'm being dense, but I don't see a problem there. You _should_
>>>call the new mapping sync page if it has been migrated.
>>
>>But can some other thread calling lock_page first find the old mapping,
>>and then run its ->sync_page which finds the new mapping? While it may
>>not matter for anyone in-tree, it does break the API so it would be
>>better to either fix it or rip it out than be silently buggy.
>
>
> Splicing a page from one mapping to another is rather worrying/exciting,
> but it does look safely done to me. remove_mapping checks page_count
> while page lock and old mapping->tree_lock are held, and gives up if
> anyone else has an interest in the page. And we already know it's
> unsafe to lock_page without holding a reference to the page, don't we?

Oh, that's true. I had thought that splice allows stealing pages with
an elevated refcount, which Jens was thinking about at one stage. But
I see that code isn't in mainline. AFAIKS it would allow other
->pin()ers to attempt to lock the page while it was being stolen.

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SUSE Labs, Novell Inc.
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