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Date: Sat, 30 Mar 2024 09:25:18 +0100
From: Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To: Dexuan Cui <decui@microsoft.com>
Cc: hch@lst.de, m.szyprowski@samsung.com, robin.murphy@arm.com,
 zhangpeng362@huawei.com, iommu@lists.linux.dev, mhklinux@outlook.com,
 linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] swiotlb: Do not set total_used to 0 in
 swiotlb_create_debugfs_files()
Message-ID: <20240330092518.71b5f0c0@meshulam.tesarici.cz>
In-Reply-To: <20240329192809.17318-1-decui@microsoft.com>
References: <20240329192809.17318-1-decui@microsoft.com>
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V Fri, 29 Mar 2024 12:28:09 -0700
Dexuan Cui <decui@microsoft.com> naps=C3=A1no:

> Sometimes the readout of /sys/kernel/debug/swiotlb/io_tlb_used and
> io_tlb_used_hiwater can be a huge number (e.g. 18446744073709551615),
> which is actually a negative number if we use "%ld" to print the number.
>=20
> When swiotlb_create_default_debugfs() is running from late_initcall,
> mem->total_used may already be non-zero, because the storage driver
> may have already started to perform I/O operations: if the storage
> driver is built-in, its probe() callback is called before late_initcall.
>=20
> swiotlb_create_debugfs_files() should not blindly set mem->total_used
> and mem->used_hiwater to 0; actually it doesn't have to initialize the
> fields at all, because the fields, as part of the global struct
> io_tlb_default_mem, have been implicitly initialized to zero.
>=20
> Also don't explicitly set mem->transient_nslabs to 0.
>=20
> Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water ma=
rks in debugfs")
> Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient poo=
l usage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

FWIW I confirm that the transient_nslabs counter does not have to be
initialized either.

Reviewed-by: Petr Tesarik <petr.tesarik1@huawei-partners.com>

Petr T