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Subject[PATCH 04/25] iov_iter: Add I/O discard iterator [ver #2]
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Add a new iterator, ITER_DISCARD, that can only be used in READ mode and
just discards any data copied to it.

This is useful in a network filesystem for discarding any unwanted data
sent by a server.

Signed-off-by: David Howells <dhowells@redhat.com>
---

include/linux/uio.h | 7 ++++++
lib/iov_iter.c | 55 +++++++++++++++++++++++++++++++++++++++++++++------
2 files changed, 55 insertions(+), 7 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 3d8f1acc142c..55ce99ddb912 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -26,6 +26,7 @@ enum iter_type {
ITER_KVEC = 2,
ITER_BVEC = 4,
ITER_PIPE = 8,
+ ITER_DISCARD = 16,
};

struct iov_iter {
@@ -72,6 +73,11 @@ static inline bool iov_iter_is_pipe(const struct iov_iter *i)
return iov_iter_type(i) == ITER_PIPE;
}

+static inline bool iov_iter_is_discard(const struct iov_iter *i)
+{
+ return iov_iter_type(i) == ITER_DISCARD;
+}
+
static inline unsigned char iov_iter_rw(const struct iov_iter *i)
{
return i->type & (READ | WRITE);
@@ -220,6 +226,7 @@ void iov_iter_bvec(struct iov_iter *i, unsigned int direction, const struct bio_
unsigned long nr_segs, size_t count);
void iov_iter_pipe(struct iov_iter *i, unsigned int direction, struct pipe_inode_info *pipe,
size_t count);
+void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count);
ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
size_t maxsize, unsigned maxpages, size_t *start);
ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index c42b928b15ef..7ebccb5c1637 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -83,6 +83,7 @@
const struct kvec *kvec; \
struct kvec v; \
iterate_kvec(i, n, v, kvec, skip, (K)) \
+ } else if (unlikely(i->type & ITER_DISCARD)) { \
} else { \
const struct iovec *iov; \
struct iovec v; \
@@ -114,6 +115,8 @@
} \
i->nr_segs -= kvec - i->kvec; \
i->kvec = kvec; \
+ } else if (unlikely(i->type & ITER_DISCARD)) { \
+ skip += n; \
} else { \
const struct iovec *iov; \
struct iovec v; \
@@ -838,7 +841,9 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
kunmap_atomic(kaddr);
return wanted;
- } else if (likely(!iov_iter_is_pipe(i)))
+ } else if (unlikely(iov_iter_is_discard(i)))
+ return bytes;
+ else if (likely(!iov_iter_is_pipe(i)))
return copy_page_to_iter_iovec(page, offset, bytes, i);
else
return copy_page_to_iter_pipe(page, offset, bytes, i);
@@ -850,7 +855,7 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
{
if (unlikely(!page_copy_sane(page, offset, bytes)))
return 0;
- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
WARN_ON(1);
return 0;
}
@@ -910,7 +915,7 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
kunmap_atomic(kaddr);
return 0;
}
- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
kunmap_atomic(kaddr);
WARN_ON(1);
return 0;
@@ -978,6 +983,10 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
pipe_advance(i, size);
return;
}
+ if (unlikely(iov_iter_is_discard(i))) {
+ i->count -= size;
+ return;
+ }
iterate_and_advance(i, size, v, 0, 0, 0)
}
EXPORT_SYMBOL(iov_iter_advance);
@@ -1013,6 +1022,8 @@ void iov_iter_revert(struct iov_iter *i, size_t unroll)
pipe_truncate(i);
return;
}
+ if (unlikely(iov_iter_is_discard(i)))
+ return;
if (unroll <= i->iov_offset) {
i->iov_offset -= unroll;
return;
@@ -1055,6 +1066,8 @@ size_t iov_iter_single_seg_count(const struct iov_iter *i)
return i->count; // it is a silly place, anyway
if (i->nr_segs == 1)
return i->count;
+ if (unlikely(iov_iter_is_discard(i)))
+ return i->count;
else if (iov_iter_is_bvec(i))
return min(i->count, i->bvec->bv_len - i->iov_offset);
else
@@ -1103,6 +1116,24 @@ void iov_iter_pipe(struct iov_iter *i, unsigned int direction,
}
EXPORT_SYMBOL(iov_iter_pipe);

+/**
+ * iov_iter_discard - Initialise an I/O iterator that discards data
+ * @i: The iterator to initialise.
+ * @direction: The direction of the transfer.
+ * @count: The size of the I/O buffer in bytes.
+ *
+ * Set up an I/O iterator that just discards everything that's written to it.
+ * It's only available as a READ iterator.
+ */
+void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count)
+{
+ BUG_ON(direction != READ);
+ i->type = ITER_DISCARD | READ;
+ i->count = count;
+ i->iov_offset = 0;
+}
+EXPORT_SYMBOL(iov_iter_discard);
+
unsigned long iov_iter_alignment(const struct iov_iter *i)
{
unsigned long res = 0;
@@ -1127,7 +1158,7 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
unsigned long res = 0;
size_t size = i->count;

- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
WARN_ON(1);
return ~0U;
}
@@ -1197,6 +1228,9 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,

if (unlikely(iov_iter_is_pipe(i)))
return pipe_get_pages(i, pages, maxsize, maxpages, start);
+ if (unlikely(iov_iter_is_discard(i)))
+ return -EFAULT;
+
iterate_all_kinds(i, maxsize, v, ({
unsigned long addr = (unsigned long)v.iov_base;
size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
@@ -1274,6 +1308,9 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,

if (unlikely(iov_iter_is_pipe(i)))
return pipe_get_pages_alloc(i, pages, maxsize, start);
+ if (unlikely(iov_iter_is_discard(i)))
+ return -EFAULT;
+
iterate_all_kinds(i, maxsize, v, ({
unsigned long addr = (unsigned long)v.iov_base;
size_t len = v.iov_len + (*start = addr & (PAGE_SIZE - 1));
@@ -1315,7 +1352,7 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
__wsum sum, next;
size_t off = 0;
sum = *csum;
- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
WARN_ON(1);
return 0;
}
@@ -1357,7 +1394,7 @@ bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum,
__wsum sum, next;
size_t off = 0;
sum = *csum;
- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
WARN_ON(1);
return false;
}
@@ -1402,7 +1439,7 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, __wsum *csum,
__wsum sum, next;
size_t off = 0;
sum = *csum;
- if (unlikely(iov_iter_is_pipe(i))) {
+ if (unlikely(iov_iter_is_pipe(i) || iov_iter_is_discard(i))) {
WARN_ON(1); /* for now */
return 0;
}
@@ -1444,6 +1481,8 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)

if (!size)
return 0;
+ if (unlikely(iov_iter_is_discard(i)))
+ return 0;

if (unlikely(iov_iter_is_pipe(i))) {
struct pipe_inode_info *pipe = i->pipe;
@@ -1487,6 +1526,8 @@ const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
WARN_ON(1);
return NULL;
}
+ if (unlikely(iov_iter_is_discard(new)))
+ return NULL;
if (iov_iter_is_bvec(new))
return new->bvec = kmemdup(new->bvec,
new->nr_segs * sizeof(struct bio_vec),
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