don't merge bios over iomap boundaries, was: Re: [PATCH] erofs: prevent buffered read bio merges across device chunks

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jun 12, 2026 at 11:42:38AM +0800, Gao Xiang wrote:
> > Reported-by: Kelu Ye <yekelu1@huawei.com>
> > Assisted-by: Codex:GPT-5.5
> > Signed-off-by: Yifan Zhao <zhaoyifan28@huawei.com>
> 
> I think it's an iomap bug instead, see:
> 
> iomap_bio_read_folio_range(), we should fix iomap instead.

Yes.  iomap should not try to build bios over iomap boundaries.
caused various issues.  Ritesh ran into that with the ext2 port
back in the day, and I actually ran into it again with an under
development xfs feature.

Can you try this patch?

---
From 297230cc3c08cbfef3670b08c4e35813c18c523e Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Sun, 7 Jun 2026 08:53:20 +0200
Subject: iomap: submit read bio after each extent

This keeps bios from crossing RTG boundaries in XFS and probably fixes
all kinds of other stuff..

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/iomap/buffered-io.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d55b936e6986..3642a11c102f 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -597,12 +597,13 @@ void iomap_read_folio(const struct iomap_ops *ops,
 
 	trace_iomap_readpage(iter.inode, 1);
 
-	while ((ret = iomap_iter(&iter, ops)) > 0)
+	while ((ret = iomap_iter(&iter, ops)) > 0) {
 		iter.status = iomap_read_folio_iter(&iter, ctx,
 				&bytes_submitted);
-
-	if (ctx->read_ctx && ctx->ops->submit_read)
-		ctx->ops->submit_read(&iter, ctx);
+		if (ctx->read_ctx && ctx->ops->submit_read)
+			ctx->ops->submit_read(&iter, ctx);
+		ctx->read_ctx = NULL;
+	}
 
 	if (ctx->cur_folio)
 		iomap_read_end(ctx->cur_folio, bytes_submitted);
@@ -664,12 +665,13 @@ void iomap_readahead(const struct iomap_ops *ops,
 
 	trace_iomap_readahead(rac->mapping->host, readahead_count(rac));
 
-	while (iomap_iter(&iter, ops) > 0)
+	while (iomap_iter(&iter, ops) > 0) {
 		iter.status = iomap_readahead_iter(&iter, ctx,
 					&cur_bytes_submitted);
-
-	if (ctx->read_ctx && ctx->ops->submit_read)
-		ctx->ops->submit_read(&iter, ctx);
+		if (ctx->read_ctx && ctx->ops->submit_read)
+			ctx->ops->submit_read(&iter, ctx);
+		ctx->read_ctx = NULL;
+	}
 
 	if (ctx->cur_folio)
 		iomap_read_end(ctx->cur_folio, cur_bytes_submitted);
-- 
2.53.0