Date:   Thu, 26 Jan 2023 16:54:07 -0800
From:   Isaac Manjarres <isaacmanjarres@google.com>
To:     Rob Herring <robh@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Frank Rowand <frowand.list@gmail.com>, linux-mm@kvack.org,
        Saravana Kannan <saravanak@google.com>, stable@vge.kernel.org,
        Calvin Zhang <calvinzhang.cool@gmail.com>,
        kernel-team@android.com, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1] Revert "mm: kmemleak: alloc gray object for reserved
 region with direct map"
Message-ID: <Y9Mgr0NhaTWCsXYz@google.com>
References: <20230124230254.295589-1-isaacmanjarres@google.com>
 <20230125210215.GA2879887-robh@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230125210215.GA2879887-robh@kernel.org>
Precedence: bulk

On Wed, Jan 25, 2023 at 03:02:15PM -0600, Rob Herring wrote:
> On Tue, Jan 24, 2023 at 03:02:54PM -0800, Isaac J. Manjarres wrote:
> > This reverts commit 972fa3a7c17c9d60212e32ecc0205dc585b1e769.
> > 
> > Kmemleak operates by periodically scanning memory regions for pointers
> > to allocated memory blocks to determine if they are leaked or not.
> > However, reserved memory regions can be used for DMA transactions
> > between a device and a CPU, and thus, wouldn't contain pointers to
> > allocated memory blocks, making them inappropriate for kmemleak to
> > scan. Thus, revert this commit.
> 
> Both commits talk about the same type of memory as CMA is used for DMA. 
> Neither commit tells me what are the implications of kmemleak scanning 
> or not scanning the region? Are there false positives?
If kmemleak scans the region when CONFIG_DEBUG_PAGEALLOC is enabled,
and the CMA region is not in use, then kmemleak will cause a page-fault,
because the region is unmapped from the kernel's address space. kmemleak
scans the region because it thinks it is in use/allocated, when it's
not. While this sounds like a problem of kmemleak not being informed
about the region not being actually allocated, it's also a matter of CMA
regions not being suitable for kmemleak to scan in the first place,
because they don't have pointers to other memory blocks; they might have
data from DMA transactions with another device, so it doesn't make
sense for kmemleak to scan these areas.

--Isaac