Re: [RFC PATCH v2 2/5] iomap: Add initial support for buffered RWF_WRITETHROUGH

[Ojaswin Mujoo]

On Mon, Apr 20, 2026 at 01:56:02PM +0200, Pankaj Raghav (Samsung) wrote:
> > +
> > +	if (wt_ops->writethrough_submit)
> > +		wt_ops->writethrough_submit(wt_ctx->inode, iomap, wt_ctx->bio_pos,
> > +					    len);
> > +
> > +	bio = bio_alloc(iomap->bdev, wt_ctx->nr_bvecs, REQ_OP_WRITE, GFP_NOFS);
> 
> We might want to check if bio_alloc succeeded here.

Hi Pankaj, so we pass GFP_NOFS which has GFP_DIRECT_RECLAIM and
according to comment over bio_alloc()

 * If %__GFP_DIRECT_RECLAIM is set then bio_alloc will always be able to
 * allocate a bio.  This is due to the mempool guarantees.  To make this work,
 * callers must never allocate more than 1 bio at a time from the general pool.

And we seem to be following this.

> 
> > +	bio->bi_iter.bi_sector	= iomap_sector(iomap, wt_ctx->bio_pos);
> > +	bio->bi_end_io		= iomap_writethrough_bio_end_io;
> > +	bio->bi_private		= wt_ctx;
> > +
> > +	for (i = 0; i < wt_ctx->nr_bvecs; i++)
> > +		__bio_add_page(bio, wt_ctx->bvec[i].bv_page,
> > +				wt_ctx->bvec[i].bv_len,
> > +				wt_ctx->bvec[i].bv_offset);
> > +
> > +	atomic_inc(&wt_ctx->ref);
> > +	submit_bio(bio);
> > +	wt_ctx->nr_bvecs = 0;
> > +}
> > +
> <snip>
> > +
> > +/**
> > + * iomap_writethrough_iter - perform RWF_WRITETHROUGH buffered write
> > + * @wt_ctx: writethrough context
> > + * @iter: iomap iter holding mapping information
> > + * @i: iov_iter for write
> > + * @wt_ops: the fs callbacks needed for writethrough
> > + *
> > + * This function copies the user buffer to folio similar to usual buffered
> > + * IO path, with the difference that we immediately issue the IO. For this we
> > + * utilize IO submission and completion mechanism that is inspired by dio.
> > + *
> > + * Folio handling note: We might be writing through a partial folio so we need
> > + * to be careful to not clear the folio dirty bit unless there are no dirty blocks
> > + * in the folio after the writethrough.
> > + */
> > +static int iomap_writethrough_iter(struct iomap_writethrough_ctx *wt_ctx,
> > +				   struct iomap_iter *iter, struct iov_iter *i,
> > +				   const struct iomap_writethrough_ops *wt_ops)
> > +
> > +{
> > +	ssize_t total_written = 0;
> > +	int status = 0;
> > +	struct address_space *mapping = iter->inode->i_mapping;
> > +	size_t chunk = mapping_max_folio_size(mapping);
> > +	unsigned int bdp_flags = (iter->flags & IOMAP_NOWAIT) ? BDP_ASYNC : 0;
> > +	unsigned int bs = i_blocksize(iter->inode);
> > +
> > +	/* copied over based on DIO handles these flags */
> > +	if (iter->iomap.type == IOMAP_UNWRITTEN)
> > +		wt_ctx->flags |= IOMAP_DIO_UNWRITTEN;
> > +	if (iter->iomap.flags & IOMAP_F_SHARED)
> > +		wt_ctx->flags |= IOMAP_DIO_COW;
> > +
> > +	if (!(iter->flags & IOMAP_WRITETHROUGH))
> > +		return -EINVAL;
> > +
> > +	do {
> > +		struct folio *folio;
> > +		size_t offset;		/* Offset into folio */
> > +		u64 bytes;		/* Bytes to write to folio */
> > +		size_t copied;		/* Bytes copied from user */
> > +		u64 written;		/* Bytes have been written */
> > +		loff_t pos;
> > +		size_t off_aligned, len_aligned;
> > +
> > +		bytes = iov_iter_count(i);
> > +retry:
> > +		offset = iter->pos & (chunk - 1);
> > +		bytes = min(chunk - offset, bytes);
> > +		status = balance_dirty_pages_ratelimited_flags(mapping,
> > +							       bdp_flags);
> > +		if (unlikely(status))
> > +			break;
> > +
> > +		/*
> > +		 * If completions already occurred and reported errors, give up
> > +		 * now and don't bother submitting more bios.
> > +		 */
> > +		if (unlikely(data_race(wt_ctx->error))) {
> 
> In the unlikely scenario where we encounter an error, do we have to also
> clear the writeback flag on all the folios that is part of this
> bvec until now?
> 
> Something like explicitly iterate over wt_ctx->bvec[0] through 
> wt_ctx->bvec[nr_bvecs - 1], manually call folio_end_writeback(bvec[i].bv_page)
> on them, and then discard the bvecs by setting the nr_bvecs = 0;
> 
> I am wondering if the folios that were processed until now will be in
> PG_WRITEBACK state which can affect reclaim as we never clear the flag.

Hey Pankaj, yes you are right. I think the error handling is a bit buggy
and Sashiko has also pointed some of these. I'll take care of this in
v3, thanks for pointing this out.

Regards,
ojaswin

> 
> > +			wt_ctx->nr_bvecs = 0;
> > +			break;
> > +		}
> > +
> 
> --
> Pankaj