Re: [PATCH v6 4/5] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Mike Snitzer <snitzer@kernel.org>]

On Thu, Oct 23, 2025 at 03:37:36PM -0400, Chuck Lever wrote:
> On 10/22/25 3:22 PM, Chuck Lever wrote:
> > +struct nfsd_write_dio {
> > +	ssize_t	start_len;	/* Length for misaligned first extent */
> > +	ssize_t	middle_len;	/* Length for DIO-aligned middle extent */
> > +	ssize_t	end_len;	/* Length for misaligned last extent */
> > +};
> > +
> > +static bool
> > +nfsd_is_write_dio_possible(loff_t offset, unsigned long len,
> > +			   struct nfsd_file *nf,
> > +			   struct nfsd_write_dio *write_dio)
> > +{
> > +	const u32 dio_blocksize = nf->nf_dio_offset_align;
> > +	loff_t start_end, orig_end, middle_end;
> > +
> > +	if (unlikely(dio_blocksize > PAGE_SIZE || len < dio_blocksize))
> > +		return false;
> > +
> > +	start_end = round_up(offset, dio_blocksize);
> > +	orig_end = offset + len;
> > +	middle_end = round_down(orig_end, dio_blocksize);
> > +
> > +	write_dio->start_len = start_end - offset;
> > +	write_dio->middle_len = middle_end - start_end;
> > +	write_dio->end_len = orig_end - middle_end;
> > +
> > +	return true;
> > +}
> > +
> > +static bool
> > +nfsd_iov_iter_aligned_bvec(const struct iov_iter *i, unsigned int addr_mask,
> > +			   unsigned int len_mask)
> > +{
> > +	const struct bio_vec *bvec = i->bvec;
> > +	size_t skip = i->iov_offset;
> > +	size_t size = i->count;
> > +
> > +	if (size & len_mask)
> > +		return false;
> > +	do {
> > +		size_t len = bvec->bv_len;
> > +
> > +		if (len > size)
> > +			len = size;
> > +		if ((unsigned long)(bvec->bv_offset + skip) & addr_mask)
> > +			return false;
> > +		bvec++;
> > +		size -= len;
> > +		skip = 0;
> > +	} while (size);
> > +
> > +	return true;
> > +}
> > +
> 
> Hey Mike, I'm trying to understand when nfsd_is_write_dio_possible()
> would return true but nfsd_iov_iter_aligned_bvec() on the middle segment
> would return false.

It is always due to memory alignment (addr_mask check), never due to
logical alignment (len_mask check).

So we could remove the len_mask arg and the 'if (size & len_mask)'
check from nfsd_iov_iter_aligned_bvec