Re: [PATCH v2 01/15] block: Make bio_set_ioprio() modify fewer bio->bi_ioprio bits

[Kanchan Joshi <joshi.k@samsung.com>]

On 10/11/2023 10:22 PM, Bart Van Assche wrote:
>>> @@ -2926,7 +2926,8 @@ static void bio_set_ioprio(struct bio *bio)
>>>    {
>>>        /* Nobody set ioprio so far? Initialize it based on task's 
>>> nice value */
>>>        if (IOPRIO_PRIO_CLASS(bio->bi_ioprio) == IOPRIO_CLASS_NONE)
>>> -        bio->bi_ioprio = get_current_ioprio();
>>> +        ioprio_set_class_and_level(&bio->bi_ioprio,
>>> +                       get_current_ioprio());
>>>        blkcg_set_ioprio(bio);
>>>    }
>>> diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
>>> index 7578d4f6a969..f2e768ab4b35 100644
>>> --- a/include/linux/ioprio.h
>>> +++ b/include/linux/ioprio.h
>>> @@ -71,4 +71,14 @@ static inline int ioprio_check_cap(int ioprio)
>>>    }
>>>    #endif /* CONFIG_BLOCK */
>>> +#define IOPRIO_CLASS_LEVEL_MASK ((IOPRIO_CLASS_MASK << 
>>> IOPRIO_CLASS_SHIFT) | \
>>> +                 (IOPRIO_LEVEL_MASK << 0))
>>> +
>>> +static inline void ioprio_set_class_and_level(u16 *prio, u16 
>>> class_level)
>>> +{
>>> +    WARN_ON_ONCE(class_level & ~IOPRIO_CLASS_LEVEL_MASK);
>>> +    *prio &= ~IOPRIO_CLASS_LEVEL_MASK;
>>> +    *prio |= class_level;
>>
>> Return of get_current_ioprio() will touch all 16 bits here. So
>> user-defined value can alter whatever was set in bio by F2FS (patch 4 in
>> this series). Is that not an issue?
> 
> The above is incomprehensible to me. Anyway, I will try to answer.
> 
> It is not clear to me why it is claimed that "get_current_ioprio() will
> touch all 16 bits here"? The return value of get_current_ioprio() is
> passed to ioprio_set_class_and_level() and that function clears the hint
> bits from the get_current_ioprio() return value.

Function does OR bio->bi_ioprio with whatever is the return of 
get_current_ioprio(). So if lifetime bits were set in 
get_current_ioprio(), you will end up setting that in bio->bi_ioprio too.


> ioprio_set_class_and_level() preserves the hint bits set by F2FS.
> 
>> And what is the user interface you have in mind. Is it ioprio based, or
>> write-hint based or mix of both?
> 
> Since the data lifetime is encoded in the hint bits, the hint bits need
> to be set by user space to set a data lifetime.

I asked because more than one way seems to emerge here. Parts of this 
series (Patch 4) are taking inode->i_write_hint (and not ioprio value) 
and putting that into bio.
I wonder what to expect if application get to send one lifetime with 
fcntl (write-hints) and different one with ioprio. Is that not racy?


> In case you would help,
> the blktest test that I wrote to test this functionality is available
> below.
> 
> Thanks,
> 
> Bart.
> 
> 
> diff --git a/tests/scsi/097 b/tests/scsi/097
> new file mode 100755
> index 000000000000..01d280021653
> --- /dev/null
> +++ b/tests/scsi/097
> @@ -0,0 +1,62 @@
> +#!/bin/bash
> +# SPDX-License-Identifier: GPL-3.0+
> +# Copyright (C) 2022 Google LLC
> +
> +. tests/zbd/rc
> +. common/null_blk
> +. common/scsi_debug
> +
> +DESCRIPTION="test block data lifetime support"
> +QUICK=1
> +
> +requires() {
> +    _have_fio
> +    _have_module scsi_debug
> +}
> +
> +test() {
> +    echo "Running ${TEST_NAME}"
> +
> +    local scsi_debug_params=(
> +        delay=0
> +        dev_size_mb=1024
> +        sector_size=4096
> +    )
> +    _init_scsi_debug "${scsi_debug_params[@]}" &&
> +    local dev="/dev/${SCSI_DEBUG_DEVICES[0]}" fail &&
> +    ls -ld "${dev}" >>"${FULL}" &&
> +    local i fio_args=(
> +        --group_reporting=1
> +        --gtod_reduce=1
> +        --ioscheduler=none
> +        --norandommap
> +        --direct=1
> +        --rw=randwrite
> +        --ioengine=io_uring
> +        --time_based
> +    ) &&
> +    for ((i=1; i<=3; i++)); do
> +        fio_args+=(
> +            --name=whint"$i"
> +            --filename="${dev}"
> +            --prio=$((i<<6))

This will not work as prio can only take values between 0-7.
Perhaps you want to use "priohint" to send lifetime.