linux-kernel - Re: Can kfree() sleep at runtime?

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Date:   Fri, 1 Jun 2018 09:12:20 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Can kfree() sleep at runtime?



On 2018/5/31 22:08, Matthew Wilcox wrote:
> On Thu, May 31, 2018 at 09:10:07PM +0800, Jia-Ju Bai wrote:
>> I write a static analysis tool (DSAC), and it finds that kfree() can sleep.
>>
>> Here is the call path for kfree().
>> Please look at it *from the bottom up*.
>>
>> [FUNC] alloc_pages(GFP_KERNEL)
>> arch/x86/mm/pageattr.c, 756: alloc_pages in split_large_page
>> arch/x86/mm/pageattr.c, 1283: split_large_page in __change_page_attr
> Here's your bug.  Coming from kfree(), we can't end up in the
> split_large_page() path.  __change_page_attr may be called in several
> different circumstances in which it would have to split a large page,
> but the path from kfree() is not one of them.
>
> I think the path from kfree() will lead to the 'level == PG_LEVEL_4K'
> path, but I'm not really familiar with this x86 code.

Thanks for reply :)
But from the code in my call path, I cannot find why kfree() will only lead to the 'level == PG_LEVEL_4K' path.
Could you please explain it in more detail?


Best wishes,
Jia-Ju Bai