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

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Date:   Thu, 31 May 2018 14:09:30 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Can kfree() sleep at runtime?

On Thu, 31 May 2018, Jia-Ju Bai wrote:

> I write a static analysis tool (DSAC), and it finds that kfree() can sleep.

That should not happen.

> 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
> arch/x86/mm/pageattr.c, 1391: __change_page_attr in __change_page_attr_set_clr
> arch/x86/mm/pageattr.c, 2014: __change_page_attr_set_clr in __set_pages_np
> arch/x86/mm/pageattr.c, 2034: __set_pages_np in __kernel_map_pages
> ./include/linux/mm.h, 2488: __kernel_map_pages in kernel_map_pages
> mm/page_alloc.c, 1074: kernel_map_pages in free_pages_prepare

mapping pages in the page allocator can cause allocations?? How did that
get in there?

> mm/page_alloc.c, 1264: free_pages_prepare in __free_pages_ok
> mm/page_alloc.c, 4312: __free_pages_ok in __free_pages
> mm/slub.c, 3914: __free_pages in kfree
>
> I always have an impression that kfree() never sleeps, so I feel confused
> here.

Correct.