Re: [PATCH v5 1/2] iomap: fix zero padding data issue in concurrent append writes

[Long Li <leo.lilong@huawei.com>]

On Wed, Dec 04, 2024 at 07:17:45AM -0500, Brian Foster wrote:
> > Coming back to our current issue, during writeback mapping, we sample
> > the inode size to determine if the ioend is within EOF and attempt to
> > trim io_size. Concurrent truncate operations may update the inode size,
> > causing the pos of write back beyond EOF. In such cases, we simply don't
> > trim io_size, which seems like a viable approach.
> > 
> 
> Perhaps. I'm not claiming it isn't functional. But to Dave's (more
> elaborated) point and in light of the racy i_size issue you've
> uncovered, what bugs me also about this is that this creates an internal
> inconsistency in the submission codepath.
> 
> I.e., the top level code does one thing based on one value of i_size,
> then the ioend construction does another, and the logic is not directly
> correlated so there is no real guarantee changes in one area correlate
> to the other. IME, this increases potential for future bugs and adds
> maintenance burden.
> 
> A simple example to consider might be.. suppose sometime in the future
> we determine there is a selective case where we do want to allow a
> post-eof writeback. As of right now, all that really requires is
> adjustment to the "handle_eof()" logic and the rest of the codepath does
> the right thing agnostic to outside operations like truncate. I think
> there's value if we can preserve that invariant going forward.
> 
> FWIW, I'm not objecting to the alternative if something in the above
> reasoning is wrong. I'm just trying to prioritize keeping things simple
> and maintainable, particularly since truncate is kind of a complicated
> beast as it is.
> 
> Brian
> 

Yes, I agree with you, thanks for the detailed explanation.

Thanks,
Long Li