On Thu, Jul 15, 2021 at 01:59:17PM -0700, Darrick J. Wong wrote:

On Thu, Jul 15, 2021 at 04:36:16AM +0100, Matthew Wilcox (Oracle) wrote:

quoted

This is a thin wrapper around bio_add_page().  The main advantage here
is the documentation that the submitter can expect to see folios in the
completion handler, and that stupidly large folios are not supported.
It's not currently possible to allocate stupidly large folios, but if
it ever becomes possible, this function will fail gracefully instead of
doing I/O to the wrong bytes.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 block/bio.c         | 21 +++++++++++++++++++++
 include/linux/bio.h |  3 ++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 1fab762e079b..1b500611d25c 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -933,6 +933,27 @@ int bio_add_page(struct bio *bio, struct page *page,
 }
 EXPORT_SYMBOL(bio_add_page);
 
+/**
+ * bio_add_folio - Attempt to add part of a folio to a bio.
+ * @bio: Bio to add to.
+ * @folio: Folio to add.
+ * @len: How many bytes from the folio to add.
+ * @off: First byte in this folio to add.
+ *
+ * Always uses the head page of the folio in the bio.  If a submitter
+ * only uses bio_add_folio(), it can count on never seeing tail pages
+ * in the completion routine.  BIOs do not support folios larger than 2GiB.
+ *
+ * Return: The number of bytes from this folio added to the bio.
+ */
+size_t bio_add_folio(struct bio *bio, struct folio *folio, size_t len,
+		size_t off)
+{
+	if (len > UINT_MAX || off > UINT_MAX)

Er... if bios don't support folios larger than 2GB, then why check @off
and @len against UINT_MAX, which is ~4GB?

I suppose that's mostly a documentation problem.  The limit is:

struct bio_vec {
        struct page     *bv_page;
        unsigned int    bv_len;
        unsigned int    bv_offset;
};

so we can support folios which are 2GB in size (0x8000'0000) bytes, but
if (theoretically, some day) we had a 4GB folio, we wouldn't be able to
handle it in a single bio_vec.  So there isn't anything between a 2GB
folio and a 4GB folio; a 4GB folio isn't allowed and a 2GB one is.

I don't think anything's wrong here, but maybe things could be worded
better?