introduce copy_page_to_iter, kill loop over iovec in generic_file_aio_read()

generic_file_aio_read() was looping over the target iovec, with loop over
(source) pages nested inside that.  Just set an iov_iter up and pass *that*
to do_generic_file_aio_read().  With copy_page_to_iter() doing all work
of mapping and copying a page to iovec and advancing iov_iter.

Switch shmem_file_aio_read() to the same and kill file_read_actor(), while
we are at it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 136 insertions, 143 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index bfb7a97d6d0f..a16eb2c4f316 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1085,11 +1085,90 @@ static void shrink_readahead_size_eio(struct file *filp,
         ra->ra_pages /= 4;
 }
 
+size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
+                         struct iov_iter *i)
+{
+        size_t skip, copy, left, wanted;
+        const struct iovec *iov;
+        char __user *buf;
+        void *kaddr, *from;
+
+        if (unlikely(bytes > i->count))
+                bytes = i->count;
+
+        if (unlikely(!bytes))
+                return 0;
+
+        wanted = bytes;
+        iov = i->iov;
+        skip = i->iov_offset;
+        buf = iov->iov_base + skip;
+        copy = min(bytes, iov->iov_len - skip);
+
+        if (!fault_in_pages_writeable(buf, copy)) {
+                kaddr = kmap_atomic(page);
+                from = kaddr + offset;
+
+                /* first chunk, usually the only one */
+                left = __copy_to_user_inatomic(buf, from, copy);
+                copy -= left;
+                skip += copy;
+                from += copy;
+                bytes -= copy;
+
+                while (unlikely(!left && bytes)) {
+                        iov++;
+                        buf = iov->iov_base;
+                        copy = min(bytes, iov->iov_len);
+                        left = __copy_to_user_inatomic(buf, from, copy);
+                        copy -= left;
+                        skip = copy;
+                        from += copy;
+                        bytes -= copy;
+                }
+                if (likely(!bytes)) {
+                        kunmap_atomic(kaddr);
+                        goto done;
+                }
+                offset = from - kaddr;
+                buf += copy;
+                kunmap_atomic(kaddr);
+                copy = min(bytes, iov->iov_len - skip);
+        }
+        /* Too bad - revert to non-atomic kmap */
+        kaddr = kmap(page);
+        from = kaddr + offset;
+        left = __copy_to_user(buf, from, copy);
+        copy -= left;
+        skip += copy;
+        from += copy;
+        bytes -= copy;
+        while (unlikely(!left && bytes)) {
+                iov++;
+                buf = iov->iov_base;
+                copy = min(bytes, iov->iov_len);
+                left = __copy_to_user(buf, from, copy);
+                copy -= left;
+                skip = copy;
+                from += copy;
+                bytes -= copy;
+        }
+        kunmap(page);
+done:
+        i->count -= wanted - bytes;
+        i->nr_segs -= iov - i->iov;
+        i->iov = iov;
+        i->iov_offset = skip;
+        return wanted - bytes;
+}
+EXPORT_SYMBOL(copy_page_to_iter);
+
 /**
  * do_generic_file_read - generic file read routine
  * @filp:       the file to read
  * @ppos:       current file position
- * @desc:       read_descriptor
+ * @iter:       data destination
+ * @written:    already copied
  *
  * This is a generic file read routine, and uses the
  * mapping->a_ops->readpage() function for the actual low-level stuff.
@@ -1097,8 +1176,8 @@ static void shrink_readahead_size_eio(struct file *filp,
  * This is really ugly. But the goto's actually try to clarify some
  * of the logic when it comes to error handling etc.
  */
-static void do_generic_file_read(struct file *filp, loff_t *ppos,
-                read_descriptor_t *desc)
+static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
+                struct iov_iter *iter, ssize_t written)
 {
         struct address_space *mapping = filp->f_mapping;
         struct inode *inode = mapping->host;
@@ -1108,12 +1187,12 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
         pgoff_t prev_index;
         unsigned long offset;      /* offset into pagecache page */
         unsigned int prev_offset;
-        int error;
+        int error = 0;
 
         index = *ppos >> PAGE_CACHE_SHIFT;
         prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
         prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
-        last_index = (*ppos + desc->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
+        last_index = (*ppos + iter->count + PAGE_CACHE_SIZE-1) >> PAGE_CACHE_SHIFT;
         offset = *ppos & ~PAGE_CACHE_MASK;
 
         for (;;) {
@@ -1148,7 +1227,7 @@ find_page:
                         if (!page->mapping)
                                 goto page_not_up_to_date_locked;
                         if (!mapping->a_ops->is_partially_uptodate(page,
-                                                        offset, desc->count))
+                                                        offset, iter->count))
                                 goto page_not_up_to_date_locked;
                         unlock_page(page);
                 }
@@ -1198,24 +1277,23 @@ page_ok:
                 /*
                  * Ok, we have the page, and it's up-to-date, so
                  * now we can copy it to user space...
-                 *
-                 * The file_read_actor routine returns how many bytes were
-                 * actually used..
-                 * NOTE! This may not be the same as how much of a user buffer
-                 * we filled up (we may be padding etc), so we can only update
-                 * "pos" here (the actor routine has to update the user buffer
-                 * pointers and the remaining count).
                  */
-                ret = file_read_actor(desc, page, offset, nr);
+
+                ret = copy_page_to_iter(page, offset, nr, iter);
                 offset += ret;
                 index += offset >> PAGE_CACHE_SHIFT;
                 offset &= ~PAGE_CACHE_MASK;
                 prev_offset = offset;
 
                 page_cache_release(page);
-                if (ret == nr && desc->count)
-                        continue;
-                goto out;
+                written += ret;
+                if (!iov_iter_count(iter))
+                        goto out;
+                if (ret < nr) {
+                        error = -EFAULT;
+                        goto out;
+                }
+                continue;
 
 page_not_up_to_date:
                 /* Get exclusive access to the page ... */
@@ -1250,6 +1328,7 @@ readpage:
                 if (unlikely(error)) {
                         if (error == AOP_TRUNCATED_PAGE) {
                                 page_cache_release(page);
+                                error = 0;
                                 goto find_page;
                         }
                         goto readpage_error;
@@ -1280,7 +1359,6 @@ readpage:
 
 readpage_error:
                 /* UHHUH! A synchronous read error occurred. Report it */
-                desc->error = error;
                 page_cache_release(page);
                 goto out;
 
@@ -1291,16 +1369,17 @@ no_cached_page:
                  */
                 page = page_cache_alloc_cold(mapping);
                 if (!page) {
-                        desc->error = -ENOMEM;
+                        error = -ENOMEM;
                         goto out;
                 }
                 error = add_to_page_cache_lru(page, mapping,
                                                 index, GFP_KERNEL);
                 if (error) {
                         page_cache_release(page);
-                        if (error == -EEXIST)
+                        if (error == -EEXIST) {
+                                error = 0;
                                 goto find_page;
-                        desc->error = error;
+                        }
                         goto out;
                 }
                 goto readpage;
@@ -1313,44 +1392,7 @@ out:
 
         *ppos = ((loff_t)index << PAGE_CACHE_SHIFT) + offset;
         file_accessed(filp);
-}
-
-int file_read_actor(read_descriptor_t *desc, struct page *page,
-                        unsigned long offset, unsigned long size)
-{
-        char *kaddr;
-        unsigned long left, count = desc->count;
-
-        if (size > count)
-                size = count;
-
-        /*
-         * Faults on the destination of a read are common, so do it before
-         * taking the kmap.
-         */
-        if (!fault_in_pages_writeable(desc->arg.buf, size)) {
-                kaddr = kmap_atomic(page);
-                left = __copy_to_user_inatomic(desc->arg.buf,
-                                                kaddr + offset, size);
-                kunmap_atomic(kaddr);
-                if (left == 0)
-                        goto success;
-        }
-
-        /* Do it the slow way */
-        kaddr = kmap(page);
-        left = __copy_to_user(desc->arg.buf, kaddr + offset, size);
-        kunmap(page);
-
-        if (left) {
-                size -= left;
-                desc->error = -EFAULT;
-        }
-success:
-        desc->count = count - size;
-        desc->written += size;
-        desc->arg.buf += size;
-        return size;
+        return written ? written : error;
 }
 
 /*
@@ -1408,14 +1450,15 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 {
         struct file *filp = iocb->ki_filp;
         ssize_t retval;
-        unsigned long seg = 0;
         size_t count;
         loff_t *ppos = &iocb->ki_pos;
+        struct iov_iter i;
 
         count = 0;
         retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
         if (retval)
                 return retval;
+        iov_iter_init(&i, iov, nr_segs, count, 0);
 
         /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
         if (filp->f_flags & O_DIRECT) {
@@ -1437,6 +1480,11 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
                 if (retval > 0) {
                         *ppos = pos + retval;
                         count -= retval;
+                        /*
+                         * If we did a short DIO read we need to skip the
+                         * section of the iov that we've already read data into.
+                         */
+                        iov_iter_advance(&i, retval);
                 }
 
                 /*
@@ -1453,39 +1501,7 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
                 }
         }
 
-        count = retval;
-        for (seg = 0; seg < nr_segs; seg++) {
-                read_descriptor_t desc;
-                loff_t offset = 0;
-
-                /*
-                 * If we did a short DIO read we need to skip the section of the
-                 * iov that we've already read data into.
-                 */
-                if (count) {
-                        if (count > iov[seg].iov_len) {
-                                count -= iov[seg].iov_len;
-                                continue;
-                        }
-                        offset = count;
-                        count = 0;
-                }
-
-                desc.written = 0;
-                desc.arg.buf = iov[seg].iov_base + offset;
-                desc.count = iov[seg].iov_len - offset;
-                if (desc.count == 0)
-                        continue;
-                desc.error = 0;
-                do_generic_file_read(filp, ppos, &desc);
-                retval += desc.written;
-                if (desc.error) {
-                        retval = retval ?: desc.error;
-                        break;
-                }
-                if (desc.count > 0)
-                        break;
-        }
+        retval = do_generic_file_read(filp, ppos, &i, retval);
 out:
         return retval;
 }
diff --git a/mm/shmem.c b/mm/shmem.c
index 9398e6cd48cb..17d3799d04bd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1462,13 +1462,25 @@ shmem_write_end(struct file *file, struct address_space *mapping,
         return copied;
 }
 
-static void do_shmem_file_read(struct file *filp, loff_t *ppos, read_descriptor_t *desc)
+static ssize_t shmem_file_aio_read(struct kiocb *iocb,
+                const struct iovec *iov, unsigned long nr_segs, loff_t pos)
 {
-        struct inode *inode = file_inode(filp);
+        struct file *file = iocb->ki_filp;
+        struct inode *inode = file_inode(file);
         struct address_space *mapping = inode->i_mapping;
         pgoff_t index;
         unsigned long offset;
         enum sgp_type sgp = SGP_READ;
+        int error;
+        ssize_t retval;
+        size_t count;
+        loff_t *ppos = &iocb->ki_pos;
+        struct iov_iter iter;
+
+        retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
+        if (retval)
+                return retval;
+        iov_iter_init(&iter, iov, nr_segs, count, 0);
 
         /*
          * Might this read be for a stacking filesystem?  Then when reading
@@ -1496,10 +1508,10 @@ static void do_shmem_file_read(struct file *filp, loff_t *ppos, read_descriptor_
                                 break;
                 }
 
-                desc->error = shmem_getpage(inode, index, &page, sgp, NULL);
-                if (desc->error) {
-                        if (desc->error == -EINVAL)
-                                desc->error = 0;
+                error = shmem_getpage(inode, index, &page, sgp, NULL);
+                if (error) {
+                        if (error == -EINVAL)
+                                error = 0;
                         break;
                 }
                 if (page)
@@ -1543,61 +1555,26 @@ static void do_shmem_file_read(struct file *filp, loff_t *ppos, read_descriptor_
                 /*
                  * Ok, we have the page, and it's up-to-date, so
                  * now we can copy it to user space...
-                 *
-                 * The actor routine returns how many bytes were actually used..
-                 * NOTE! This may not be the same as how much of a user buffer
-                 * we filled up (we may be padding etc), so we can only update
-                 * "pos" here (the actor routine has to update the user buffer
-                 * pointers and the remaining count).
                  */
-                ret = file_read_actor(desc, page, offset, nr);
+                ret = copy_page_to_iter(page, offset, nr, &iter);
+                retval += ret;
                 offset += ret;
                 index += offset >> PAGE_CACHE_SHIFT;
                 offset &= ~PAGE_CACHE_MASK;
 
                 page_cache_release(page);
-                if (ret != nr || !desc->count)
+                if (!iov_iter_count(&iter))
                         break;
-
+                if (ret < nr) {
+                        error = -EFAULT;
+                        break;
+                }
                 cond_resched();
         }
 
         *ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
-        file_accessed(filp);
-}
-
-static ssize_t shmem_file_aio_read(struct kiocb *iocb,
-                const struct iovec *iov, unsigned long nr_segs, loff_t pos)
-{
-        struct file *filp = iocb->ki_filp;
-        ssize_t retval;
-        unsigned long seg;
-        size_t count;
-        loff_t *ppos = &iocb->ki_pos;
-
-        retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
-        if (retval)
-                return retval;
-
-        for (seg = 0; seg < nr_segs; seg++) {
-                read_descriptor_t desc;
-
-                desc.written = 0;
-                desc.arg.buf = iov[seg].iov_base;
-                desc.count = iov[seg].iov_len;
-                if (desc.count == 0)
-                        continue;
-                desc.error = 0;
-                do_shmem_file_read(filp, ppos, &desc);
-                retval += desc.written;
-                if (desc.error) {
-                        retval = retval ?: desc.error;
-                        break;
-                }
-                if (desc.count > 0)
-                        break;
-        }
-        return retval;
+        file_accessed(file);
+        return retval ? retval : error;
 }
 
 static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,