iov_iter: Add I/O discard iterator

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>

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 3d8f1acc142c2a3b4a700b0b84f20af9965588d9..55ce99ddb912f9bd603e5031a328eba96e37fc78 100644 (file)
--- 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 c42b928b15efa158c9e473286fc72698d9f539fc..7ebccb5c16377eb77176d0df689e1cfb7af7b36d 100644 (file)
--- 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),