[PATCH] Add support for the sys_vmsplice syscall

sys_splice() moves data to/from pipes with a file input/output. sys_vmsplice()
moves data to a pipe, with the input being a user address range instead.

This uses an approach suggested by Linus, where we can hold partial ranges
inside the pages[] map. Hopefully this will be useful for network
receive support as well.

Signed-off-by: Jens Axboe <axboe@suse.de>

1 files changed, 253 insertions, 39 deletions

diff --git a/fs/splice.c b/fs/splice.c
index 8c6030c762e2..0b2c1f060cae 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -27,6 +27,7 @@
 #include <linux/buffer_head.h>
 #include <linux/module.h>
 #include <linux/syscalls.h>
+#include <linux/uio.h>
 
 /*
  * Passed to the actors
@@ -38,6 +39,22 @@ struct splice_desc {
         loff_t pos;                     /* file position */
 };
 
+struct partial_page {
+        unsigned int offset;
+        unsigned int len;
+};
+
+/*
+ * Passed to move_to_pipe
+ */
+struct splice_pipe_desc {
+        struct page **pages;            /* page map */
+        struct partial_page *partial;   /* pages[] may not be contig */
+        int nr_pages;                   /* number of pages in map */
+        unsigned int flags;             /* splice flags */
+        struct pipe_buf_operations *ops;/* ops associated with output pipe */
+};
+
 /*
  * Attempt to steal a page from a pipe buffer. This should perhaps go into
  * a vm helper function, it's already simplified quite a bit by the
@@ -128,6 +145,19 @@ static void page_cache_pipe_buf_unmap(struct pipe_inode_info *info,
         kunmap(buf->page);
 }
 
+static void *user_page_pipe_buf_map(struct file *file,
+                                    struct pipe_inode_info *pipe,
+                                    struct pipe_buffer *buf)
+{
+        return kmap(buf->page);
+}
+
+static void user_page_pipe_buf_unmap(struct pipe_inode_info *pipe,
+                                     struct pipe_buffer *buf)
+{
+        kunmap(buf->page);
+}
+
 static void page_cache_pipe_buf_get(struct pipe_inode_info *info,
                                     struct pipe_buffer *buf)
 {
@@ -143,19 +173,33 @@ static struct pipe_buf_operations page_cache_pipe_buf_ops = {
         .get = page_cache_pipe_buf_get,
 };
 
+static int user_page_pipe_buf_steal(struct pipe_inode_info *pipe,
+                                    struct pipe_buffer *buf)
+{
+        return 1;
+}
+
+static struct pipe_buf_operations user_page_pipe_buf_ops = {
+        .can_merge = 0,
+        .map = user_page_pipe_buf_map,
+        .unmap = user_page_pipe_buf_unmap,
+        .release = page_cache_pipe_buf_release,
+        .steal = user_page_pipe_buf_steal,
+        .get = page_cache_pipe_buf_get,
+};
+
 /*
  * Pipe output worker. This sets up our pipe format with the page cache
  * pipe buffer operations. Otherwise very similar to the regular pipe_writev().
  */
-static ssize_t move_to_pipe(struct pipe_inode_info *pipe, struct page **pages,
-                            int nr_pages, unsigned long len,
-                            unsigned int offset, unsigned int flags)
+static ssize_t move_to_pipe(struct pipe_inode_info *pipe,
+                            struct splice_pipe_desc *spd)
 {
-        int ret, do_wakeup, i;
+        int ret, do_wakeup, page_nr;
 
         ret = 0;
         do_wakeup = 0;
-        i = 0;
+        page_nr = 0;
 
         if (pipe->inode)
                 mutex_lock(&pipe->inode->i_mutex);
@@ -171,27 +215,19 @@ static ssize_t move_to_pipe(struct pipe_inode_info *pipe, struct page **pages,
                 if (pipe->nrbufs < PIPE_BUFFERS) {
                         int newbuf = (pipe->curbuf + pipe->nrbufs) & (PIPE_BUFFERS - 1);
                         struct pipe_buffer *buf = pipe->bufs + newbuf;
-                        struct page *page = pages[i++];
-                        unsigned long this_len;
 
-                        this_len = PAGE_CACHE_SIZE - offset;
-                        if (this_len > len)
-                                this_len = len;
-
-                        buf->page = page;
-                        buf->offset = offset;
-                        buf->len = this_len;
-                        buf->ops = &page_cache_pipe_buf_ops;
+                        buf->page = spd->pages[page_nr];
+                        buf->offset = spd->partial[page_nr].offset;
+                        buf->len = spd->partial[page_nr].len;
+                        buf->ops = spd->ops;
                         pipe->nrbufs++;
+                        page_nr++;
+                        ret += buf->len;
+
                         if (pipe->inode)
                                 do_wakeup = 1;
 
-                        ret += this_len;
-                        len -= this_len;
-                        offset = 0;
-                        if (!--nr_pages)
-                                break;
-                        if (!len)
+                        if (!--spd->nr_pages)
                                 break;
                         if (pipe->nrbufs < PIPE_BUFFERS)
                                 continue;
@@ -199,7 +235,7 @@ static ssize_t move_to_pipe(struct pipe_inode_info *pipe, struct page **pages,
                         break;
                 }
 
-                if (flags & SPLICE_F_NONBLOCK) {
+                if (spd->flags & SPLICE_F_NONBLOCK) {
                         if (!ret)
                                 ret = -EAGAIN;
                         break;
@@ -234,8 +270,8 @@ static ssize_t move_to_pipe(struct pipe_inode_info *pipe, struct page **pages,
                 kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
         }
 
-        while (i < nr_pages)
-                page_cache_release(pages[i++]);
+        while (page_nr < spd->nr_pages)
+                page_cache_release(spd->pages[page_nr++]);
 
         return ret;
 }
@@ -246,17 +282,24 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
                            unsigned int flags)
 {
         struct address_space *mapping = in->f_mapping;
-        unsigned int loff, offset, nr_pages;
+        unsigned int loff, nr_pages;
         struct page *pages[PIPE_BUFFERS];
+        struct partial_page partial[PIPE_BUFFERS];
         struct page *page;
         pgoff_t index, end_index;
         loff_t isize;
-        size_t bytes;
-        int i, error;
+        size_t total_len;
+        int error;
+        struct splice_pipe_desc spd = {
+                .pages = pages,
+                .partial = partial,
+                .flags = flags,
+                .ops = &page_cache_pipe_buf_ops,
+        };
 
         index = *ppos >> PAGE_CACHE_SHIFT;
-        loff = offset = *ppos & ~PAGE_CACHE_MASK;
-        nr_pages = (len + offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
+        loff = *ppos & ~PAGE_CACHE_MASK;
+        nr_pages = (len + loff + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
 
         if (nr_pages > PIPE_BUFFERS)
                 nr_pages = PIPE_BUFFERS;
@@ -266,15 +309,15 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
          * read-ahead if this is a non-zero offset (we are likely doing small
          * chunk splice and the page is already there) for a single page.
          */
-        if (!offset || nr_pages > 1)
-                do_page_cache_readahead(mapping, in, index, nr_pages);
+        if (!loff || spd.nr_pages > 1)
+                do_page_cache_readahead(mapping, in, index, spd.nr_pages);
 
         /*
          * Now fill in the holes:
          */
         error = 0;
-        bytes = 0;
-        for (i = 0; i < nr_pages; i++, index++) {
+        total_len = 0;
+        for (spd.nr_pages = 0; spd.nr_pages < nr_pages; spd.nr_pages++, index++) {
                 unsigned int this_len;
 
                 if (!len)
@@ -367,26 +410,29 @@ readpage:
                          */
                         if (end_index == index) {
                                 loff = PAGE_CACHE_SIZE - (isize & ~PAGE_CACHE_MASK);
-                                if (bytes + loff > isize) {
+                                if (total_len + loff > isize) {
                                         page_cache_release(page);
                                         break;
                                 }
                                 /*
                                  * force quit after adding this page
                                  */
-                                nr_pages = i;
+                                nr_pages = spd.nr_pages;
                                 this_len = min(this_len, loff);
+                                loff = 0;
                         }
                 }
 fill_it:
-                pages[i] = page;
-                bytes += this_len;
+                pages[spd.nr_pages] = page;
+                partial[spd.nr_pages].offset = loff;
+                partial[spd.nr_pages].len = this_len;
                 len -= this_len;
+                total_len += this_len;
                 loff = 0;
         }
 
-        if (i)
-                return move_to_pipe(pipe, pages, i, bytes, offset, flags);
+        if (spd.nr_pages)
+                return move_to_pipe(pipe, &spd);
 
         return error;
 }
@@ -1018,6 +1064,174 @@ static long do_splice(struct file *in, loff_t __user *off_in,
         return -EINVAL;
 }
 
+/*
+ * Map an iov into an array of pages and offset/length tupples. With the
+ * partial_page structure, we can map several non-contiguous ranges into
+ * our ones pages[] map instead of splitting that operation into pieces.
+ * Could easily be exported as a generic helper for other users, in which
+ * case one would probably want to add a 'max_nr_pages' parameter as well.
+ */
+static int get_iovec_page_array(const struct iovec __user *iov,
+                                unsigned int nr_vecs, struct page **pages,
+                                struct partial_page *partial)
+{
+        int buffers = 0, error = 0;
+
+        /*
+         * It's ok to take the mmap_sem for reading, even
+         * across a "get_user()".
+         */
+        down_read(&current->mm->mmap_sem);
+
+        while (nr_vecs) {
+                unsigned long off, npages;
+                void __user *base;
+                size_t len;
+                int i;
+
+                /*
+                 * Get user address base and length for this iovec.
+                 */
+                error = get_user(base, &iov->iov_base);
+                if (unlikely(error))
+                        break;
+                error = get_user(len, &iov->iov_len);
+                if (unlikely(error))
+                        break;
+
+                /*
+                 * Sanity check this iovec. 0 read succeeds.
+                 */
+                if (unlikely(!len))
+                        break;
+                error = -EFAULT;
+                if (unlikely(!base))
+                        break;
+
+                /*
+                 * Get this base offset and number of pages, then map
+                 * in the user pages.
+                 */
+                off = (unsigned long) base & ~PAGE_MASK;
+                npages = (off + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
+                if (npages > PIPE_BUFFERS - buffers)
+                        npages = PIPE_BUFFERS - buffers;
+
+                error = get_user_pages(current, current->mm,
+                                       (unsigned long) base, npages, 0, 0,
+                                       &pages[buffers], NULL);
+
+                if (unlikely(error <= 0))
+                        break;
+
+                /*
+                 * Fill this contiguous range into the partial page map.
+                 */
+                for (i = 0; i < error; i++) {
+                        const int plen = min_t(size_t, len, PAGE_SIZE) - off;
+
+                        partial[buffers].offset = off;
+                        partial[buffers].len = plen;
+
+                        off = 0;
+                        len -= plen;
+                        buffers++;
+                }
+
+                /*
+                 * We didn't complete this iov, stop here since it probably
+                 * means we have to move some of this into a pipe to
+                 * be able to continue.
+                 */
+                if (len)
+                        break;
+
+                /*
+                 * Don't continue if we mapped fewer pages than we asked for,
+                 * or if we mapped the max number of pages that we have
+                 * room for.
+                 */
+                if (error < npages || buffers == PIPE_BUFFERS)
+                        break;
+
+                nr_vecs--;
+                iov++;
+        }
+
+        up_read(&current->mm->mmap_sem);
+
+        if (buffers)
+                return buffers;
+
+        return error;
+}
+
+/*
+ * vmsplice splices a user address range into a pipe. It can be thought of
+ * as splice-from-memory, where the regular splice is splice-from-file (or
+ * to file). In both cases the output is a pipe, naturally.
+ *
+ * Note that vmsplice only supports splicing _from_ user memory to a pipe,
+ * not the other way around. Splicing from user memory is a simple operation
+ * that can be supported without any funky alignment restrictions or nasty
+ * vm tricks. We simply map in the user memory and fill them into a pipe.
+ * The reverse isn't quite as easy, though. There are two possible solutions
+ * for that:
+ *
+ *      - memcpy() the data internally, at which point we might as well just
+ *        do a regular read() on the buffer anyway.
+ *      - Lots of nasty vm tricks, that are neither fast nor flexible (it
+ *        has restriction limitations on both ends of the pipe).
+ *
+ * Alas, it isn't here.
+ *
+ */
+static long do_vmsplice(struct file *file, const struct iovec __user *iov,
+                        unsigned long nr_segs, unsigned int flags)
+{
+        struct pipe_inode_info *pipe = file->f_dentry->d_inode->i_pipe;
+        struct page *pages[PIPE_BUFFERS];
+        struct partial_page partial[PIPE_BUFFERS];
+        struct splice_pipe_desc spd = {
+                .pages = pages,
+                .partial = partial,
+                .flags = flags,
+                .ops = &user_page_pipe_buf_ops,
+        };
+
+        if (unlikely(!pipe))
+                return -EBADF;
+        if (unlikely(nr_segs > UIO_MAXIOV))
+                return -EINVAL;
+        else if (unlikely(!nr_segs))
+                return 0;
+
+        spd.nr_pages = get_iovec_page_array(iov, nr_segs, pages, partial);
+        if (spd.nr_pages <= 0)
+                return spd.nr_pages;
+
+        return move_to_pipe(pipe, &spd);
+}
+
+asmlinkage long sys_vmsplice(int fd, const struct iovec __user *iov,
+                             unsigned long nr_segs, unsigned int flags)
+{
+        struct file *file;
+        long error;
+        int fput;
+
+        error = -EBADF;
+        file = fget_light(fd, &fput);
+        if (file) {
+                if (file->f_mode & FMODE_WRITE)
+                        error = do_vmsplice(file, iov, nr_segs, flags);
+
+                fput_light(file, fput);
+        }
+
+        return error;
+}
+
 asmlinkage long sys_splice(int fd_in, loff_t __user *off_in,
                            int fd_out, loff_t __user *off_out,
                            size_t len, unsigned int flags)