linux-kernel - Re: [PATCH] mm: kmemleak: Ignore kmemleak scanning on CMA regions

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Date:   Fri, 3 Dec 2021 08:52:24 +0800
From:   Calvin Zhang <calvinzhang.cool@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] mm: kmemleak: Ignore kmemleak scanning on CMA regions

On Thu, Dec 02, 2021 at 06:11:12PM +0000, Catalin Marinas wrote:
>On Sun, Nov 28, 2021 at 09:50:53AM +0800, Calvin Zhang wrote:
>> On Sat, Nov 27, 2021 at 04:07:18PM -0800, Andrew Morton wrote:
>> >On Fri, 26 Nov 2021 10:47:11 +0800 Calvin Zhang <calvinzhang.cool@...il.com> wrote:
>> >> Just like this:
>> >> commit 620951e27457 ("mm/cma: make kmemleak ignore CMA regions").
>> >> 
>> >> Add kmemleak_ignore_phys() for CMA created from of reserved node.
>[...]
>> >The 620951e27457 changelog says "Without this, the kernel crashes...". 
>> >Does your patch also fix a crash?  If so under what circumstances and
>> >should we backport this fix into -stable kernels?
>> 
>> No crash occurred. 620951e27457 avoids crashes caused by accessing
>> highmem and it was fixed later. Now kmemleak_alloc_phys() and
>> kmemleak_ignore_phys() skip highmem. This patch is based on the
>> point that CMA regions don't contain pointers to other kmemleak
>> objects, and ignores CMA regions from reserved memory as what
>> 620951e27457 did.
>
>Note that kmemleak_ignore() only works if there was a prior
>kmemleak_alloc() on that address range. With the previous commit we get
>this via the memblock_alloc_range() but I fail to see one on the
>rmem_cma_setup() path.

rmem is from memblock_reserve() or early_init_dt_alloc_reserved_memory_arch()
kmemleak_alloc() is not called in the first case. And It's bad to add one.

I think all the reserved regions should be allocated from memblock without
kmemleak_alloc() and let rmem handler choose to add it as kmemleak object
by kmemleak_alloc(). Because MEMBLOCK_ALLOC_NOLEAKTRACE conflicts with range
parameter in memlbock_alloc_* series, all reserved regions and default CMA
region are allocated with kmemleak_alloc().

I think it's better to add memblock_alloc_* series a spearate flag paramter
(like "NOLEAKTRACE") instead of encoding MEMBLOCK_ALLOC_NOLEAKTRACE in `end`
parameter.

--
Calvin