From: Bart Van Assche <bvanassche@acm.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
Ming Lei <ming.lei@redhat.com>,
Bart Van Assche <bvanassche@acm.org>,
Christoph Hellwig <hch@infradead.org>
Subject: [PATCH 3/5] block: Document the bio_vec properties
Date: Fri, 15 May 2020 17:19:12 -0700 [thread overview]
Message-ID: <20200516001914.17138-4-bvanassche@acm.org> (raw)
In-Reply-To: <20200516001914.17138-1-bvanassche@acm.org>
Since it is nontrivial that nth_page() does not have to be used for a
bio_vec, document this.
CC: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
include/linux/bvec.h | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index a81c13ac1972..25295c11b164 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -12,8 +12,22 @@
#include <linux/errno.h>
#include <linux/mm.h>
-/*
- * was unsigned short, but we might as well be ready for > 64kB I/O pages
+/**
+ * struct bio_vec - a contiguous range of physical memory addresses
+ * @bv_page: First page associated with the address range.
+ * @bv_len: Number of bytes in the address range.
+ * @bv_offset: Start of the address range relative to the start of @bv_page.
+ *
+ * The following holds for a bvec if n * PAGE_SIZE < bv_offset + bv_len:
+ *
+ * nth_page(@bv_page, n) == @bv_page + n
+ *
+ * From the description of commit 52d52d1c98a9 ("block: only allow contiguous
+ * page structs in a bio_vec"): "We currently have to call nth_page when
+ * iterating over pages inside a bio_vec. Jens complained a while ago that
+ * this is fairly expensive. To mitigate this we can check that that the
+ * actual page structures are contiguous when adding them to the bio, and just
+ * do check pointer arithmetics later on." See also page_is_mergeable().
*/
struct bio_vec {
struct page *bv_page;
next prev parent reply other threads:[~2020-05-16 0:19 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-16 0:19 [PATCH 0/5] Block layer patches for kernel v5.8 Bart Van Assche
2020-05-16 0:19 ` [PATCH 1/5] block: Fix type of first compat_put_{,u}long() argument Bart Van Assche
2020-05-16 7:40 ` Arnd Bergmann
2020-05-16 12:41 ` Christoph Hellwig
2020-05-16 0:19 ` [PATCH 2/5] bio.h: Declare the arguments of bio iteration functions const Bart Van Assche
2020-05-16 8:55 ` Alexander Potapenko
2020-05-16 17:10 ` Bart Van Assche
2020-05-16 12:42 ` Christoph Hellwig
2020-05-17 22:46 ` Chaitanya Kulkarni
2020-05-16 0:19 ` Bart Van Assche [this message]
2020-05-16 12:43 ` [PATCH 3/5] block: Document the bio_vec properties Christoph Hellwig
2020-05-16 0:19 ` [PATCH 4/5] block: Fix zero_fill_bio() Bart Van Assche
2020-05-16 5:50 ` Ming Lei
2020-05-16 0:19 ` [PATCH 5/5] null_blk: Zero-initialize read buffers in non-memory-backed mode Bart Van Assche
2020-05-16 9:40 ` Alexander Potapenko
2020-05-18 1:12 ` Damien Le Moal
2020-05-18 1:32 ` Bart Van Assche
2020-05-18 2:10 ` Damien Le Moal
2020-05-18 2:56 ` Bart Van Assche
2020-05-18 3:12 ` Damien Le Moal
2020-05-18 14:31 ` Bart Van Assche
2020-05-19 3:03 ` Damien Le Moal
2020-05-19 4:10 ` Bart Van Assche
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