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Date: Thu, 31 May 2018 07:14:52 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: Can kfree() sleep at runtime?
Message-ID: <20180531141452.GC30221@bombadil.infradead.org>
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 <20180531140808.GA30221@bombadil.infradead.org>
 <01000163b68a8026-56fb6a35-040b-4af9-8b73-eb3b4a41c595-000000@email.amazonses.com>
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To: Christopher Lameter <cl@linux.com>
Cc: Jia-Ju Bai <baijiaju1990@gmail.com>, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, linux-mm@kvack.org, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

On Thu, May 31, 2018 at 02:12:00PM +0000, Christopher Lameter wrote:
> On Thu, 31 May 2018, 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.
> 
> Freeing a page in the page allocator also was traditionally not sleeping.
> That has changed?

No.  "Your bug" being "The bug in your static analysis tool".  It probably
isn't following the data flow correctly (or deeply enough).