Re: [PATCH v7 14/14] NFSD: Initialize separate ki_flags

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 27, 2025 at 11:26:58PM -0400, Mike Snitzer wrote:
> But none of that matters if the only safe way to implement mixing
> buffered and direct IO is by waiting for the DIO to succeed and with
> it any associated page cache invalidation (and associated possible
> failure to invalidate the page handled with using buffered IO fallback
> by underlying filesystem).

Why do you think it is the only safe way?  (The above seems to imply
that to, or maybe it is a question?)

> 
> Any buffered or direct IO associated with the misaligned DIO WRITE
> handling, in terms of 3 segments, must use IOCB_DSYNC.

Can you explain why exactly.  If that's is indeed the case, it is a
very important point we clearly need to document.

> But the entire intent behind NFSD's O_DIRECT support is to ensure IO
> is on stable storage when it replies to the client.

Is it?  This is the first time I read that this is the "entire point".
I though the main reason was to stop wasting memory on the server
and reduce memory copies.  If the entire intent is to to commit to
stable storage only, there is no need for all the direct I/O games,
and you can just use RWF_DSYNC only.

> The client isn't
> meant to get involved with driving the correctness of NFSD's O_DIRECT
> support (by requiring the client set NFS_FILE_SYNC for the benefit of
> a feature it doesn't know enabled in the server).

Of course the client should not care about the servers implementation
detail.  But I don't see how this is relevant here at all.

> IOCB_DIRECT without IOCB_DSYNC isn't an option because we must ensure
> the data is ondisk.

Why?

> The only related bake-off would be:
> 1) IOCB_DIRECT | IOCB_DSYNC with UNSTABLE 
>  vs
> 2) IOCB_DIRECT | IOCB_DSYNC | IOCBD_SYNC with NFS_FILE_SYNC
> 
> (but on esoteric enterprise storage: there is no difference)

What are you talking about? With any Linux file system on any storage
it does make a huge difference except for the corner case of pure
overwrites of fully allocated ranges.