linux-kernel - Re: [PATCH v2] mm/kmemleak: Avoid scanning potential huge holes

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Date:   Fri, 28 Jan 2022 19:29:26 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Lang Yu <lang.yu@....com>
Cc:     linux-mm@...ck.org, David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm/kmemleak: Avoid scanning potential huge holes

On Mon, Nov 08, 2021 at 10:00:29PM +0800, Lang Yu wrote:
> When using devm_request_free_mem_region() and devm_memremap_pages()
> to add ZONE_DEVICE memory, if requested free mem region's end pfn
> were huge(e.g., 0x400000000), the node_end_pfn() will be also huge
> (see move_pfn_range_to_zone()). Thus it creates a huge hole between
> node_start_pfn() and node_end_pfn().
> 
> We found on some AMD APUs, amdkfd requested such a free mem region
> and created a huge hole. In such a case, following code snippet was
> just doing busy test_bit() looping on the huge hole.
> 
> for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> 	struct page *page = pfn_to_online_page(pfn);
> 		if (!page)
> 			continue;
> 	...
> }
> 
> So we got a soft lockup:
> 
> watchdog: BUG: soft lockup - CPU#6 stuck for 26s! [bash:1221]
> CPU: 6 PID: 1221 Comm: bash Not tainted 5.15.0-custom #1
> RIP: 0010:pfn_to_online_page+0x5/0xd0
> Call Trace:
>   ? kmemleak_scan+0x16a/0x440
>   kmemleak_write+0x306/0x3a0
>   ? common_file_perm+0x72/0x170
>   full_proxy_write+0x5c/0x90
>   vfs_write+0xb9/0x260
>   ksys_write+0x67/0xe0
>   __x64_sys_write+0x1a/0x20
>   do_syscall_64+0x3b/0xc0
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> I did some tests with the patch.
> 
> (1) amdgpu module unloaded
> 
> before the patch:
> 
> real    0m0.976s
> user    0m0.000s
> sys     0m0.968s
> 
> after the patch:
> 
> real    0m0.981s
> user    0m0.000s
> sys     0m0.973s
> 
> (2) amdgpu module loaded
> 
> before the patch:
> 
> real    0m35.365s
> user    0m0.000s
> sys     0m35.354s
> 
> after the patch:
> 
> real    0m1.049s
> user    0m0.000s
> sys     0m1.042s
> 
> v2:
> - Only scan pages belonging to the zone.(David Hildenbrand)
> - Use __maybe_unused to make compilers happy.
> 
> Signed-off-by: Lang Yu <lang.yu@....com>

Acked-by: Catalin Marinas <catalin.marinas@....com>