[PATCH] ext4: Regression test of ext4_lblk_t overflow

[PATCH] ext4: Regression test of ext4_lblk_t overflow

[Baokun Li]

Append writes to a file approaching 16T and observe if a kernel crash is
caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
This is a regression test for commit bc056e7163ac ("ext4: fix BUG in
ext4_mb_new_inode_pa() due to overflow")

Signed-off-by: Baokun Li <libaokun1@huawei.com>
---
 tests/ext4/062     | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/ext4/062.out |  2 ++
 2 files changed, 55 insertions(+)
 create mode 100755 tests/ext4/062
 create mode 100644 tests/ext4/062.out

diff --git a/tests/ext4/062 b/tests/ext4/062
new file mode 100755
index 00000000..0ae4ed80
--- /dev/null
+++ b/tests/ext4/062
@@ -0,0 +1,53 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2023 HUAWEI.  All Rights Reserved.
+#
+# FS QA Test No. 062
+#
+# Append writes to a file approaching 16T and observe if a kernel crash is
+# caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
+# This is a regression test for commit
+# bc056e7163ac ("ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow")
+#
+. ./common/preamble
+_begin_fstest auto quick insert
+
+# real QA test starts here
+_supported_fs ext4
+_fixed_by_kernel_commit bc056e7163ac \
+	"ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow"
+
+_require_xfs_io_command "falloc"
+_require_xfs_io_command "open"
+_require_xfs_io_command "pwrite"
+_require_scratch_size_nocheck $((100 * 1024))   # 100MB in KB
+
+# Initalize a 4k 100M ext4 fs
+dev_size=$((100 * 1024 * 1024))
+MKFS_OPTIONS="-b 4096 $MKFS_OPTIONS" _scratch_mkfs_sized $dev_size \
+	>>$seqres.full 2>&1 || _fail "mkfs failed"
+
+_scratch_mount
+
+# Reserve 1M space
+$XFS_IO_PROG -f -c "falloc 0 1M" "${SCRATCH_MNT}/tmp" >> $seqres.full 2>&1
+
+# Create a file with logical block numbers close to overflow
+$XFS_IO_PROG -f -c "falloc 0 10M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
+$XFS_IO_PROG -f -c "finsert 1M 16777203M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
+
+# Filling up the free space ensures that the pre-allocated space is the reserved space.
+$XFS_IO_PROG -f -c "falloc 0 80388096" "${SCRATCH_MNT}/fill" >> $seqres.full 2>&1
+
+# Remove reserved space to gain free space for allocation
+rm -f ${SCRATCH_MNT}/tmp
+
+# Trying to allocate two blocks triggers BUG_ON.
+$XFS_IO_PROG -c "open -ad ${SCRATCH_MNT}/file" -c "pwrite -S 0xff 0 8192" >> $seqres.full 2>&1
+
+umount "${SCRATCH_MNT}"
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit
diff --git a/tests/ext4/062.out b/tests/ext4/062.out
new file mode 100644
index 00000000..a1578f48
--- /dev/null
+++ b/tests/ext4/062.out
@@ -0,0 +1,2 @@
+QA output created by 062
+Silence is golden
-- 
2.31.1

Re: [PATCH] ext4: Regression test of ext4_lblk_t overflow

[Zorro Lang]

On Sat, Oct 28, 2023 at 03:01:23PM +0800, Baokun Li wrote:
> Append writes to a file approaching 16T and observe if a kernel crash is
> caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
> This is a regression test for commit bc056e7163ac ("ext4: fix BUG in
> ext4_mb_new_inode_pa() due to overflow")
> 
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> ---

Hi Baokun,

Thanks for this new coverage, some review points as below ...


>  tests/ext4/062     | 53 ++++++++++++++++++++++++++++++++++++++++++++++
>  tests/ext4/062.out |  2 ++
>  2 files changed, 55 insertions(+)
>  create mode 100755 tests/ext4/062
>  create mode 100644 tests/ext4/062.out
> 
> diff --git a/tests/ext4/062 b/tests/ext4/062
> new file mode 100755
> index 00000000..0ae4ed80
> --- /dev/null
> +++ b/tests/ext4/062
> @@ -0,0 +1,53 @@
> +#! /bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (c) 2023 HUAWEI.  All Rights Reserved.
> +#
> +# FS QA Test No. 062
> +#
> +# Append writes to a file approaching 16T and observe if a kernel crash is
> +# caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
> +# This is a regression test for commit
> +# bc056e7163ac ("ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow")
> +#
> +. ./common/preamble
> +_begin_fstest auto quick insert
                                   ^^^
                                   prealloc

> +
> +# real QA test starts here
> +_supported_fs ext4
> +_fixed_by_kernel_commit bc056e7163ac \
> +	"ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow"
> +
> +_require_xfs_io_command "falloc"
> +_require_xfs_io_command "open"
> +_require_xfs_io_command "pwrite"

I think we don't need to check the "open" and "pwrite" operations,
if you don't need special option for them.

> +_require_scratch_size_nocheck $((100 * 1024))   # 100MB in KB

I think 100MB is small enough, don't need this check.

> +
> +# Initalize a 4k 100M ext4 fs
> +dev_size=$((100 * 1024 * 1024))
> +MKFS_OPTIONS="-b 4096 $MKFS_OPTIONS" _scratch_mkfs_sized $dev_size \
> +	>>$seqres.full 2>&1 || _fail "mkfs failed"

Can we use bsize=$(_get_file_block_size ....) to get the real block size,
to avoid use a specific mkfs option?
If so, my second question is if this case can be a generic case. The test
steps look common enough, I'm glad to see what will happen on other
filesystems through this test.

> +
> +_scratch_mount
> +
> +# Reserve 1M space
> +$XFS_IO_PROG -f -c "falloc 0 1M" "${SCRATCH_MNT}/tmp" >> $seqres.full 2>&1
> +
> +# Create a file with logical block numbers close to overflow
> +$XFS_IO_PROG -f -c "falloc 0 10M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
> +$XFS_IO_PROG -f -c "finsert 1M 16777203M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1

_require_xfs_io_command "finsert"

> +
> +# Filling up the free space ensures that the pre-allocated space is the reserved space.
> +$XFS_IO_PROG -f -c "falloc 0 80388096" "${SCRATCH_MNT}/fill" >> $seqres.full 2>&1

Can we make sure this step fill the whole free space? There's a helper in
common/populate named _fill_fs, please check if it's what you want?

> +
> +# Remove reserved space to gain free space for allocation
> +rm -f ${SCRATCH_MNT}/tmp
> +
> +# Trying to allocate two blocks triggers BUG_ON.
> +$XFS_IO_PROG -c "open -ad ${SCRATCH_MNT}/file" -c "pwrite -S 0xff 0 8192" >> $seqres.full 2>&1

Oh, you need open -d option, that means you need direct IO, so you need:
_require_odirect

And all above xfs_io commands, include falloc, finsert, open and pwrite.
You can redirect the stdout to $seqres.full, but better *not* to change
the stderr, for we can find errors if they fail (except that expected ENOSPC).

> +
> +umount "${SCRATCH_MNT}"

The $SCRATCH_MNT will be unmounted at the end of the testing, don't need to do
it manually, except this's a necessary step of the reproducer.

> +echo "Silence is golden"
> +
> +# success, all done
> +status=0
> +exit
> diff --git a/tests/ext4/062.out b/tests/ext4/062.out
> new file mode 100644
> index 00000000..a1578f48
> --- /dev/null
> +++ b/tests/ext4/062.out
> @@ -0,0 +1,2 @@
> +QA output created by 062
> +Silence is golden
> -- 
> 2.31.1
> 
> 

Re: [PATCH] ext4: Regression test of ext4_lblk_t overflow

[Baokun Li]

On 2023/11/17 22:24, Zorro Lang wrote:
> Hi Baokun,
>
> Thanks for this new coverage, some review points as below ...
>
Hi Zorro,

Thank you for your careful review！

>>   tests/ext4/062     | 53 ++++++++++++++++++++++++++++++++++++++++++++++
>>   tests/ext4/062.out |  2 ++
>>   2 files changed, 55 insertions(+)
>>   create mode 100755 tests/ext4/062
>>   create mode 100644 tests/ext4/062.out
>>
>> diff --git a/tests/ext4/062 b/tests/ext4/062
>> new file mode 100755
>> index 00000000..0ae4ed80
>> --- /dev/null
>> +++ b/tests/ext4/062
>> @@ -0,0 +1,53 @@
>> +#! /bin/bash
>> +# SPDX-License-Identifier: GPL-2.0
>> +# Copyright (c) 2023 HUAWEI.  All Rights Reserved.
>> +#
>> +# FS QA Test No. 062
>> +#
>> +# Append writes to a file approaching 16T and observe if a kernel crash is
>> +# caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
>> +# This is a regression test for commit
>> +# bc056e7163ac ("ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow")
>> +#
>> +. ./common/preamble
>> +_begin_fstest auto quick insert
>                                     ^^^
>                                     prealloc
I'll add this.
>> +
>> +# real QA test starts here
>> +_supported_fs ext4
>> +_fixed_by_kernel_commit bc056e7163ac \
>> +	"ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow"
>> +
>> +_require_xfs_io_command "falloc"
>> +_require_xfs_io_command "open"
>> +_require_xfs_io_command "pwrite"
> I think we don't need to check the "open" and "pwrite" operations,
> if you don't need special option for them.
OK!
>> +_require_scratch_size_nocheck $((100 * 1024))   # 100MB in KB
> I think 100MB is small enough, don't need this check.
All right!
>> +
>> +# Initalize a 4k 100M ext4 fs
>> +dev_size=$((100 * 1024 * 1024))
>> +MKFS_OPTIONS="-b 4096 $MKFS_OPTIONS" _scratch_mkfs_sized $dev_size \
>> +	>>$seqres.full 2>&1 || _fail "mkfs failed"
> Can we use bsize=$(_get_file_block_size ....) to get the real block size,
> to avoid use a specific mkfs option?
> If so, my second question is if this case can be a generic case. The test
> steps look common enough, I'm glad to see what will happen on other
> filesystems through this test.
The reason for specifying 4096 for formatting here is that mkfs.ext4 
specifies a
default block size of 1024 when the disk image is 100M.

Although these operations appear to be generic scenarios, the 
reproduction of
this problem relies on ext4's pre-allocation mechanism. Block allocation 
requests
are scaled up in ext4_mb_normalize_request(), and the extra blocks are 
used for
preallocation to reduce fragmentation. Therefore, this test case is 
placed only in
the ext4 test suite.
>
>> +
>> +_scratch_mount
>> +
>> +# Reserve 1M space
>> +$XFS_IO_PROG -f -c "falloc 0 1M" "${SCRATCH_MNT}/tmp" >> $seqres.full 2>&1
>> +
>> +# Create a file with logical block numbers close to overflow
>> +$XFS_IO_PROG -f -c "falloc 0 10M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
>> +$XFS_IO_PROG -f -c "finsert 1M 16777203M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
> _require_xfs_io_command "finsert"
Okay.
>
>> +
>> +# Filling up the free space ensures that the pre-allocated space is the reserved space.
>> +$XFS_IO_PROG -f -c "falloc 0 80388096" "${SCRATCH_MNT}/fill" >> $seqres.full 2>&1
> Can we make sure this step fill the whole free space? There's a helper in
> common/populate named _fill_fs, please check if it's what you want?
The remaining space for a 100M 4k ext4 image after the previous 
operations is
80388096, so we can confirm that we can fill the entire free space here.
Of course, using _fill_fs is also workable, but it's faster to do it in 
a single fallocate.
>> +
>> +# Remove reserved space to gain free space for allocation
>> +rm -f ${SCRATCH_MNT}/tmp
>> +
>> +# Trying to allocate two blocks triggers BUG_ON.
>> +$XFS_IO_PROG -c "open -ad ${SCRATCH_MNT}/file" -c "pwrite -S 0xff 0 8192" >> $seqres.full 2>&1
> Oh, you need open -d option, that means you need direct IO, so you need:
> _require_odirect

Indeed!

> And all above xfs_io commands, include falloc, finsert, open and pwrite.
> You can redirect the stdout to $seqres.full, but better *not* to change
> the stderr, for we can find errors if they fail (except that expected ENOSPC).
Ok, I'll remove the unnecessary "2>&1"!
>> +
>> +umount "${SCRATCH_MNT}"
> The $SCRATCH_MNT will be unmounted at the end of the testing, don't need to do
> it manually, except this's a necessary step of the reproducer.
Okay, I will remove it.
>
>> +echo "Silence is golden"
>> +
>> +# success, all done
>> +status=0
>> +exit
>> diff --git a/tests/ext4/062.out b/tests/ext4/062.out
>> new file mode 100644
>> index 00000000..a1578f48
>> --- /dev/null
>> +++ b/tests/ext4/062.out
>> @@ -0,0 +1,2 @@
>> +QA output created by 062
>> +Silence is golden
>> -- 
>> 2.31.1
>>
>>
>
Thanks!
-- 
With Best Regards,
Baokun Li
.

Re: [PATCH] ext4: Regression test of ext4_lblk_t overflow

[Zorro Lang]

On Sat, Nov 18, 2023 at 11:08:10AM +0800, Baokun Li wrote:
> On 2023/11/17 22:24, Zorro Lang wrote:
> > Hi Baokun,
> > 
> > Thanks for this new coverage, some review points as below ...
> > 
> Hi Zorro,
> 
> Thank you for your careful review！
> 
> > >   tests/ext4/062     | 53 ++++++++++++++++++++++++++++++++++++++++++++++
> > >   tests/ext4/062.out |  2 ++
> > >   2 files changed, 55 insertions(+)
> > >   create mode 100755 tests/ext4/062
> > >   create mode 100644 tests/ext4/062.out
> > > 
> > > diff --git a/tests/ext4/062 b/tests/ext4/062
> > > new file mode 100755
> > > index 00000000..0ae4ed80
> > > --- /dev/null
> > > +++ b/tests/ext4/062
> > > @@ -0,0 +1,53 @@
> > > +#! /bin/bash
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +# Copyright (c) 2023 HUAWEI.  All Rights Reserved.
> > > +#
> > > +# FS QA Test No. 062
> > > +#
> > > +# Append writes to a file approaching 16T and observe if a kernel crash is
> > > +# caused by ext4_lblk_t overflow triggering BUG_ON at ext4_mb_new_inode_pa().
> > > +# This is a regression test for commit
> > > +# bc056e7163ac ("ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow")
> > > +#
> > > +. ./common/preamble
> > > +_begin_fstest auto quick insert
> >                                     ^^^
> >                                     prealloc
> I'll add this.
> > > +
> > > +# real QA test starts here
> > > +_supported_fs ext4
> > > +_fixed_by_kernel_commit bc056e7163ac \
> > > +	"ext4: fix BUG in ext4_mb_new_inode_pa() due to overflow"
> > > +
> > > +_require_xfs_io_command "falloc"
> > > +_require_xfs_io_command "open"
> > > +_require_xfs_io_command "pwrite"
> > I think we don't need to check the "open" and "pwrite" operations,
> > if you don't need special option for them.
> OK!
> > > +_require_scratch_size_nocheck $((100 * 1024))   # 100MB in KB
> > I think 100MB is small enough, don't need this check.
> All right!
> > > +
> > > +# Initalize a 4k 100M ext4 fs
> > > +dev_size=$((100 * 1024 * 1024))
> > > +MKFS_OPTIONS="-b 4096 $MKFS_OPTIONS" _scratch_mkfs_sized $dev_size \
> > > +	>>$seqres.full 2>&1 || _fail "mkfs failed"
> > Can we use bsize=$(_get_file_block_size ....) to get the real block size,
> > to avoid use a specific mkfs option?
> > If so, my second question is if this case can be a generic case. The test
> > steps look common enough, I'm glad to see what will happen on other
> > filesystems through this test.
> The reason for specifying 4096 for formatting here is that mkfs.ext4
> specifies a
> default block size of 1024 when the disk image is 100M.

Is the reproducer related with the block size. If it's not, we can use
"bsize=$(_get_file_block_size $SCRATCH_MNT)" to get the block size, and change
later 4096 to $bsize.

> 
> Although these operations appear to be generic scenarios, the reproduction
> of
> this problem relies on ext4's pre-allocation mechanism. Block allocation
> requests
> are scaled up in ext4_mb_normalize_request(), and the extra blocks are used
> for
> preallocation to reduce fragmentation. Therefore, this test case is placed
> only in
> the ext4 test suite.

Sure it's a reproducer for ext4, but it can be run for other filesystems
which support falloc and finsert. And:
  _require_xfs_io_command "falloc"
  _require_xfs_io_command "finsert"
will make sure the current FSTYP supports these two features. So it can
be a bug coverage for ext4, and a generic test case for other fs.

> > 
> > > +
> > > +_scratch_mount
> > > +
> > > +# Reserve 1M space
> > > +$XFS_IO_PROG -f -c "falloc 0 1M" "${SCRATCH_MNT}/tmp" >> $seqres.full 2>&1
> > > +
> > > +# Create a file with logical block numbers close to overflow
> > > +$XFS_IO_PROG -f -c "falloc 0 10M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
> > > +$XFS_IO_PROG -f -c "finsert 1M 16777203M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
> > _require_xfs_io_command "finsert"
> Okay.
> > 
> > > +
> > > +# Filling up the free space ensures that the pre-allocated space is the reserved space.
> > > +$XFS_IO_PROG -f -c "falloc 0 80388096" "${SCRATCH_MNT}/fill" >> $seqres.full 2>&1
> > Can we make sure this step fill the whole free space? There's a helper in
> > common/populate named _fill_fs, please check if it's what you want?
> The remaining space for a 100M 4k ext4 image after the previous operations
> is
> 80388096, so we can confirm that we can fill the entire free space here.
> Of course, using _fill_fs is also workable, but it's faster to do it in a
> single fallocate.

I'm not sure how the reservation works, if there're 90M free space, and you
try to reserve 90+M, will once fallocate calling trys to reserve each free
blocks, before returning error? If it's not, I think the _fill_fs might be
what you want.

Thanks,
Zorro

> > > +
> > > +# Remove reserved space to gain free space for allocation
> > > +rm -f ${SCRATCH_MNT}/tmp
> > > +
> > > +# Trying to allocate two blocks triggers BUG_ON.
> > > +$XFS_IO_PROG -c "open -ad ${SCRATCH_MNT}/file" -c "pwrite -S 0xff 0 8192" >> $seqres.full 2>&1
> > Oh, you need open -d option, that means you need direct IO, so you need:
> > _require_odirect
> 
> Indeed!
> 
> > And all above xfs_io commands, include falloc, finsert, open and pwrite.
> > You can redirect the stdout to $seqres.full, but better *not* to change
> > the stderr, for we can find errors if they fail (except that expected ENOSPC).
> Ok, I'll remove the unnecessary "2>&1"!
> > > +
> > > +umount "${SCRATCH_MNT}"
> > The $SCRATCH_MNT will be unmounted at the end of the testing, don't need to do
> > it manually, except this's a necessary step of the reproducer.
> Okay, I will remove it.
> > 
> > > +echo "Silence is golden"
> > > +
> > > +# success, all done
> > > +status=0
> > > +exit
> > > diff --git a/tests/ext4/062.out b/tests/ext4/062.out
> > > new file mode 100644
> > > index 00000000..a1578f48
> > > --- /dev/null
> > > +++ b/tests/ext4/062.out
> > > @@ -0,0 +1,2 @@
> > > +QA output created by 062
> > > +Silence is golden
> > > -- 
> > > 2.31.1
> > > 
> > > 
> > 
> Thanks!
> -- 
> With Best Regards,
> Baokun Li
> .
> 

Re: [PATCH] ext4: Regression test of ext4_lblk_t overflow

[Baokun Li]

On 2023/11/19 21:14, Zorro Lang wrote:
> On Sat, Nov 18, 2023 at 11:08:10AM +0800, Baokun Li wrote:
>> On 2023/11/17 22:24, Zorro Lang wrote:
>>> Hi Baokun,
>>>
>>> Thanks for this new coverage, some review points as below ...
>>>
>> Hi Zorro,
>>
>> Thank you for your careful review！
>>
>>>> +
>>>> +# Initalize a 4k 100M ext4 fs
>>>> +dev_size=$((100 * 1024 * 1024))
>>>> +MKFS_OPTIONS="-b 4096 $MKFS_OPTIONS" _scratch_mkfs_sized $dev_size \
>>>> +	>>$seqres.full 2>&1 || _fail "mkfs failed"
>>> Can we use bsize=$(_get_file_block_size ....) to get the real block size,
>>> to avoid use a specific mkfs option?
>>> If so, my second question is if this case can be a generic case. The test
>>> steps look common enough, I'm glad to see what will happen on other
>>> filesystems through this test.
>> The reason for specifying 4096 for formatting here is that mkfs.ext4
>> specifies a
>> default block size of 1024 when the disk image is 100M.
> Is the reproducer related with the block size. If it's not, we can use
> "bsize=$(_get_file_block_size $SCRATCH_MNT)" to get the block size, and change
> later 4096 to $bsize.
This problem is really hard to reproduce when the block size is 1024, 
but the
good thing is that the default block size specified in 
_scratch_mkfs_sized()
is 4096, so we can leave the block size of 4096 unspecified here.
>
>> Although these operations appear to be generic scenarios, the reproduction
>> of
>> this problem relies on ext4's pre-allocation mechanism. Block allocation
>> requests
>> are scaled up in ext4_mb_normalize_request(), and the extra blocks are used
>> for
>> preallocation to reduce fragmentation. Therefore, this test case is placed
>> only in
>> the ext4 test suite.
> Sure it's a reproducer for ext4, but it can be run for other filesystems
> which support falloc and finsert. And:
>    _require_xfs_io_command "falloc"
>    _require_xfs_io_command "finsert"
> will make sure the current FSTYP supports these two features. So it can
> be a bug coverage for ext4, and a generic test case for other fs.
Totally agree, I'll put it in generic test suite in next version, the 
execution of
this use case takes only 1-2s, it doesn't seem to have any impact.
>>>> +
>>>> +_scratch_mount
>>>> +
>>>> +# Reserve 1M space
>>>> +$XFS_IO_PROG -f -c "falloc 0 1M" "${SCRATCH_MNT}/tmp" >> $seqres.full 2>&1
>>>> +
>>>> +# Create a file with logical block numbers close to overflow
>>>> +$XFS_IO_PROG -f -c "falloc 0 10M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
>>>> +$XFS_IO_PROG -f -c "finsert 1M 16777203M" "${SCRATCH_MNT}/file" >> $seqres.full 2>&1
>>> _require_xfs_io_command "finsert"
>> Okay.
>>>> +
>>>> +# Filling up the free space ensures that the pre-allocated space is the reserved space.
>>>> +$XFS_IO_PROG -f -c "falloc 0 80388096" "${SCRATCH_MNT}/fill" >> $seqres.full 2>&1
>>> Can we make sure this step fill the whole free space? There's a helper in
>>> common/populate named _fill_fs, please check if it's what you want?
>> The remaining space for a 100M 4k ext4 image after the previous operations
>> is
>> 80388096, so we can confirm that we can fill the entire free space here.
>> Of course, using _fill_fs is also workable, but it's faster to do it in a
>> single fallocate.
> I'm not sure how the reservation works, if there're 90M free space, and you
> try to reserve 90+M, will once fallocate calling trys to reserve each free
> blocks, before returning error? If it's not, I think the _fill_fs might be
> what you want.
>
> Thanks,
> Zorro
ext4_fallocate will try to allocate all free blocks if the size to be 
allocated exceeds
the free space on the filesystem, but will still return an error. I've 
tested _fill_fs and
it works well too. I will use _fill_fs in the next release to make this 
use case more generic.

Thanks!
-- 
With Best Regards,
Baokun Li
.