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[PATCH v1 02/14] block: introduce bio_remove_last_page()

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  [PATCH v1 02/14] block: introduce bio_remove_last_page() Ming Lei <tom.leiming@gmail.com> - 2017-02-24 16:50 +0100
    Re: [PATCH v1 02/14] block: introduce bio_remove_last_page() Christoph Hellwig <hch@infradead.org> - 2017-02-25 19:30 +0100
      Re: [PATCH v1 02/14] block: introduce bio_remove_last_page() Ming Lei <tom.leiming@gmail.com> - 2017-02-28 13:30 +0100

#1587736 — [PATCH v1 02/14] block: introduce bio_remove_last_page()

MD need this helper to remove the last added page, so introduce
it.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 block/bio.c         | 23 +++++++++++++++++++++++
 include/linux/bio.h |  1 +
 2 files changed, 24 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index 5eec5e08417f..0ce7ffcd7939 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -837,6 +837,29 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 EXPORT_SYMBOL(bio_add_pc_page);
 
 /**
+ *	bio_remove_last_page	-	remove the last added page
+ *	@bio: destination bio
+ *
+ *	Attempt to remove the last added page from the bio_vec maplist.
+ */
+void bio_remove_last_page(struct bio *bio)
+{
+	/*
+	 * cloned bio must not modify vec list
+	 */
+	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
+		return;
+
+	if (bio->bi_vcnt > 0) {
+		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
+
+		bio->bi_iter.bi_size -= bv->bv_len;
+		bio->bi_vcnt--;
+	}
+}
+EXPORT_SYMBOL(bio_remove_last_page);
+
+/**
  *	bio_add_page	-	attempt to add page to bio
  *	@bio: destination bio
  *	@page: page to add
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 3364b3ed90e7..32aeb493d1fe 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -443,6 +443,7 @@ extern void bio_init(struct bio *bio, struct bio_vec *table,
 extern void bio_reset(struct bio *);
 void bio_chain(struct bio *, struct bio *);
 
+extern void bio_remove_last_page(struct bio *bio);
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
 			   unsigned int, unsigned int);
-- 
2.7.4

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#1588187

On Fri, Feb 24, 2017 at 11:42:39PM +0800, Ming Lei wrote:
> MD need this helper to remove the last added page, so introduce
> it.

If MD really has a valid use case for this it should open code the
operation.  The semantics look deeply fishy to me.

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#1589401

On Sun, Feb 26, 2017 at 2:23 AM, Christoph Hellwig <hch@infradead.org> wrote:
> On Fri, Feb 24, 2017 at 11:42:39PM +0800, Ming Lei wrote:
>> MD need this helper to remove the last added page, so introduce
>> it.
>
> If MD really has a valid use case for this it should open code the
> operation.  The semantics look deeply fishy to me.

Thinking about MD's case further, and looks bio_add_page() won't
fail at all in case that queue's limit isn't applied in bio_add_page()
in future.

So I will change MD's handling in this case and avoid to introduce
bio_remove_last_page() in V2.

Thanks,
Ming Lei

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