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

Chandan Babu R
2022-03-25T06:02:56Z

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.

--
chandan