linux-kernel - Re: [PATCH v2 2/5] uprobes: __replace_page() should not use page_address_in

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Message-ID: <20120709102837.GC32393@linux.vnet.ibm.com>
Date:	Mon, 9 Jul 2012 15:58:37 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] uprobes: __replace_page() should not use
 page_address_in_vma()

> ------------------------------------------------------------------------------
> Subject: [PATCH v2 2/5] uprobes: __replace_page() should not use page_address_in_vma()
> 
> page_address_in_vma(old_page) in __replace_page() is ugly and wrong.
> The caller already knows the correct virtual address, this page was
> found by get_user_pages(vaddr).
> 
> However, page_address_in_vma() can actually fail if page->mapping was
> cleared by __delete_from_page_cache() after get_user_pages() returns.
> But this means the race with page reclaim, write_opcode() should not
> fail, it should retry and read this page again. Probably the race with
> remove_mapping() is not possible due to page_freeze_refs() logic, but
> afaics at least shmem_writepage()->shmem_delete_from_page_cache() can
> clear ->mapping.
> 
> We could change __replace_page() to return -EAGAIN in this case, but
> it would be better to simply use the caller's vaddr and rely on
> page_check_address().
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>

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