Re: [PATCH V8 15/19] xfs: Directory's data fork extent counter can never overflow

Chandan Babu R
2022-03-25T06:37:30Z

On 25 Mar 2022 at 11:32, Chandan Babu R wrote:
> On 25 Mar 2022 at 03:44, Dave Chinner wrote:
>> On Mon, Mar 21, 2022 at 10:47:46AM +0530, Chandan Babu R wrote:
>>> The maximum file size that can be represented by the data fork extent counter
>>> in the worst case occurs when all extents are 1 block in length and each block
>>> is 1KB in size.
>>>
>>> With XFS_MAX_EXTCNT_DATA_FORK_SMALL representing maximum extent count and with
>>> 1KB sized blocks, a file can reach upto,
>>> (2^31) * 1KB = 2TB
>>>
>>> This is much larger than the theoretical maximum size of a directory
>>> i.e. 32GB * 3 = 96GB.
>>>
>>> Since a directory's inode can never overflow its data fork extent counter,
>>> this commit replaces checking the return value of
>>> xfs_iext_count_may_overflow() with calls to ASSERT(error == 0).
>>
>> I'd really prefer that we don't add noise like this to a bunch of
>> call sites. If directories can't overflow the extent count in
>> normal operation, then why are we even calling
>> xfs_iext_count_may_overflow() in these paths? i.e. an overflow would
>> be a sign of an inode corruption, and we should have flagged that
>> long before we do an operation that might overflow the extent count.
>>
>> So, really, I think you should document the directory size
>> constraints at the site where we define all the large extent count
>> values in xfs_format.h, remove the xfs_iext_count_may_overflow()
>> checks from the directory code and replace them with a simple inode
>> verifier check that we haven't got more than 100GB worth of
>> individual extents in the data fork for directory inodes....
>>
>> Then all this directory specific "can't possibly overflow" overflow
>> checks can go away completely. The best code is no code :)
>
> I had retained the directory extent count overflow checks for the sake of
> completeness i.e. The code checks for overflow before every fs operation that
> could cause extent count to increase. However, I think your suggestion makes
> more sense. I will include this change in the next version of the patchset.

Also, Removing directory extent counter overflow checks would also mean that
the test xfs/533 (Verify that XFS does not cause inode fork's extent count to
overflow when adding/removing directory entries) has to be removed as well. I
will post a patch to do the same if no objections are raised.

--
chandan