[PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Dexuan Cui]

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.

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.

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.

Also don't explicitly set mem->transient_nslabs to 0.

Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water marks in debugfs")
Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient pool usage")
Cc: stable@vger.kernel.org
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
 kernel/dma/swiotlb.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 86fe172b5958..4a9f02c13da6 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1647,9 +1647,6 @@ DEFINE_DEBUGFS_ATTRIBUTE(fops_io_tlb_hiwater, io_tlb_hiwater_get,
 static void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
 					 const char *dirname)
 {
-	atomic_long_set(&mem->total_used, 0);
-	atomic_long_set(&mem->used_hiwater, 0);
-
 	mem->debugfs = debugfs_create_dir(dirname, io_tlb_default_mem.debugfs);
 	if (!mem->nslabs)
 		return;
@@ -1660,7 +1657,6 @@ static void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
 	debugfs_create_file("io_tlb_used_hiwater", 0600, mem->debugfs, mem,
 			&fops_io_tlb_hiwater);
 #ifdef CONFIG_SWIOTLB_DYNAMIC
-	atomic_long_set(&mem->transient_nslabs, 0);
 	debugfs_create_file("io_tlb_transient_nslabs", 0400, mem->debugfs,
 			    mem, &fops_io_tlb_transient_used);
 #endif
-- 
2.34.1

Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Easwar Hariharan]

On 3/29/2024 12:28 PM, Dexuan Cui wrote:
> 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.
> 
> 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.
> 
> 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.
> 
> Also don't explicitly set mem->transient_nslabs to 0.
> 
> Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water marks in debugfs")
> Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient pool usage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  kernel/dma/swiotlb.c | 4 ----
>  1 file changed, 4 deletions(-)
> 

Sorry, I'm missing a why in this commit message. Can you say what happens if the
total_used and used_hiwater IS blindly set to 0? Is the only effect the change in
the readout of the swiotlb debugfs files?

Thanks,
Easwar

RE: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Dexuan Cui]

> From: Easwar Hariharan <eahariha@linux.microsoft.com>
> Sent: Friday, March 29, 2024 12:47 PM
> [...]
> Sorry, I'm missing a why in this commit message. Can you say what happens
> if the total_used and used_hiwater IS blindly set to 0? Is the only effect
> the change  in the readout of the swiotlb debugfs files?
> 
> Thanks,
> Easwar

Right, when the system is not doing any I/O, the readout may 
return a huge number while it should return 0. This is the only effect.

Thanks,
Dexuan

RE: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Dexuan Cui]

> From: Dexuan Cui
> Sent: Friday, March 29, 2024 1:23 PM
> To: Easwar Hariharan <eahariha@linux.microsoft.com>; hch@lst.de;
> m.szyprowski@samsung.com; robin.murphy@arm.com;
> zhangpeng362@huawei.com; iommu@lists.linux.dev;
> mhklinux@outlook.com
> Cc: 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()
> 
> > From: Easwar Hariharan <eahariha@linux.microsoft.com>
> > Sent: Friday, March 29, 2024 12:47 PM
> > [...]
> > Sorry, I'm missing a why in this commit message. Can you say what
> happens
> > if the total_used and used_hiwater IS blindly set to 0? Is the only effect
> > the change  in the readout of the swiotlb debugfs files?
> >
> > Thanks,
> > Easwar
> 
> Right, when the system is not doing any I/O, the readout may
> return a huge number while it should return 0. This is the only effect.
> 
> Thanks,
> Dexuan

Let me share more details.

kernel/dma/swiotlb.c uses inc_used_and_hiwater() and dec_used()
to do the accounting.

The issue happens this way:

1. inc_used_and_hiwater() adds n to total_used.
2. swiotlb_create_debugfs_files() sets total_used to 0.
3. dec_used() decreases total_used by n, i.e. total_used incorrectly 
becomes a negative number -n, which is a huge number since
mem_used() converts the 'long' to 'unsigned long'.

Thanks,
Dexuan

Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Easwar Hariharan]

On 3/29/2024 1:30 PM, Dexuan Cui wrote:
>> From: Dexuan Cui
>> Sent: Friday, March 29, 2024 1:23 PM
>> To: Easwar Hariharan <eahariha@linux.microsoft.com>; hch@lst.de;
>> m.szyprowski@samsung.com; robin.murphy@arm.com;
>> zhangpeng362@huawei.com; iommu@lists.linux.dev;
>> mhklinux@outlook.com
>> Cc: 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()
>>
>>> From: Easwar Hariharan <eahariha@linux.microsoft.com>
>>> Sent: Friday, March 29, 2024 12:47 PM
>>> [...]
>>> Sorry, I'm missing a why in this commit message. Can you say what
>> happens
>>> if the total_used and used_hiwater IS blindly set to 0? Is the only effect
>>> the change  in the readout of the swiotlb debugfs files?
>>>
>>> Thanks,
>>> Easwar
>>
>> Right, when the system is not doing any I/O, the readout may
>> return a huge number while it should return 0. This is the only effect.
>>
>> Thanks,
>> Dexuan
> 
> Let me share more details.
> 
> kernel/dma/swiotlb.c uses inc_used_and_hiwater() and dec_used()
> to do the accounting.
> 
> The issue happens this way:
> 
> 1. inc_used_and_hiwater() adds n to total_used.
> 2. swiotlb_create_debugfs_files() sets total_used to 0.
> 3. dec_used() decreases total_used by n, i.e. total_used incorrectly 
> becomes a negative number -n, which is a huge number since
> mem_used() converts the 'long' to 'unsigned long'.
> 
> Thanks,
> Dexuan
> 

Thanks for the detail. I only ask because the patch is marked for stable, and
I was wondering if it meets the criteria. But, as you mentioned off list, two Fixes
tags probably do meet the bar.

Thanks,
Easwar

RE: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Michael Kelley]

From: Dexuan Cui <decui@microsoft.com> Sent: Friday, March 29, 2024 12:28 PM
> 
> 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.
> 
> 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.
> 
> 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.
> 
> Also don't explicitly set mem->transient_nslabs to 0.
> 
> Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water marks in debugfs")
> Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient pool usage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
>  kernel/dma/swiotlb.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 86fe172b5958..4a9f02c13da6 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1647,9 +1647,6 @@
> DEFINE_DEBUGFS_ATTRIBUTE(fops_io_tlb_hiwater, io_tlb_hiwater_get,
>  static void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
>  					 const char *dirname)
>  {
> -	atomic_long_set(&mem->total_used, 0);
> -	atomic_long_set(&mem->used_hiwater, 0);
> -
>  	mem->debugfs = debugfs_create_dir(dirname, io_tlb_default_mem.debugfs);
>  	if (!mem->nslabs)
>  		return;
> @@ -1660,7 +1657,6 @@ static void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
>  	debugfs_create_file("io_tlb_used_hiwater", 0600, mem->debugfs, mem,
>  			&fops_io_tlb_hiwater);
>  #ifdef CONFIG_SWIOTLB_DYNAMIC
> -	atomic_long_set(&mem->transient_nslabs, 0);
>  	debugfs_create_file("io_tlb_transient_nslabs", 0400, mem->debugfs,
>  			    mem, &fops_io_tlb_transient_used);
>  #endif
> --
> 2.34.1

Reviewed-by: Michael Kelley <mhklinux@outlook.com>

Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[zhangpeng (AS)]

On 2024/3/30 3:28, Dexuan Cui wrote:

> 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.
>
> 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.
>
> 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.
>
> Also don't explicitly set mem->transient_nslabs to 0.
>
> Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water marks in debugfs")
> Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient pool usage")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

Reviewed-by: ZhangPeng<zhangpeng362@huawei.com>

-- 
Best Regards,
Peng

Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Petr Tesařík]

V Fri, 29 Mar 2024 12:28:09 -0700
Dexuan Cui <decui@microsoft.com> napsáno:

> 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.
> 
> 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.
> 
> 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.
> 
> Also don't explicitly set mem->transient_nslabs to 0.
> 
> Fixes: 8b0977ecc8b3 ("swiotlb: track and report io_tlb_used high water marks in debugfs")
> Fixes: 02e765697038 ("swiotlb: add debugfs to track swiotlb transient pool 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

Re: [PATCH] swiotlb: Do not set total_used to 0 in swiotlb_create_debugfs_files()

[Christoph Hellwig]

Thanks,

applied to the dma-mapping tree for Linux 6.9-rc.