Re: [PATCH 3/4] xfs: use IOCB_DONTCACHE when falling back to buffered writes

[Christoph Hellwig]

On Tue, Nov 04, 2025 at 06:03:35PM +0530, Nirjhar Roy (IBM) wrote:
> > Doing sub-block direct writes to COW inodes is not supported by XFS,
> > because new blocks need to be allocated as a whole.  Such writes
>
> Okay, since allocation of new blocks involves whole lot of metatdata
> updates/transactions etc and that would consume a lot of time and in
> this large window the user buffer(for direct I/O) can be re-used/freed
> which would cause corruptions?

I don't understand what you're trying to say here.

> Just thinking out loud: What if we supported sub-block direct IO in XFS
> and indeed allocated new blocks+ update the metadata structures and then
> directly write the user data to the newly allocated blocks instead of
> using the page cache?
>
> Assuming the application doesn't modify the user data buffer - can we
> (at least theoritically) do such kind of sub-block DIO?

Regular XFS does that.  Zoned XFS or the always COW debug mode can't do
that (except maybe for appends) as it it requires a read-modify-write
cycle that is not implemented in iomap.  Yes, we could implement that,
but it's not going to perform any better than the fallback, and would
also require full serialization.