fuse: Implement writepages callback

The .writepages one is required to make each writeback request carry more than
one page on it. The patch enables optimized behaviour unconditionally,
i.e. mmap-ed writes will benefit from the patch even if fc->writeback_cache=0.

[SzM: simplify, add comments]

Signed-off-by: Maxim Patlasov <MPatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 6ce0066..0bd349d 100644 (file)
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1502,8 +1502,8 @@ static void fuse_writepage_end(struct fuse_conn *fc, struct fuse_req *req)
        fuse_writepage_free(fc, req);
 }
 
-static struct fuse_file *fuse_write_file(struct fuse_conn *fc,
-                                        struct fuse_inode *fi)
+static struct fuse_file *fuse_write_file_get(struct fuse_conn *fc,
+                                            struct fuse_inode *fi)
 {
        struct fuse_file *ff = NULL;
 
@@ -1540,7 +1540,7 @@ static int fuse_writepage_locked(struct page *page)
                goto err_free;
 
        error = -EIO;
-       req->ff = fuse_write_file(fc, fi);
+       req->ff = fuse_write_file_get(fc, fi);
        if (!req->ff)
                goto err_free;
 
@@ -1586,6 +1586,149 @@ static int fuse_writepage(struct page *page, struct writeback_control *wbc)
        return err;
 }
 
+struct fuse_fill_wb_data {
+       struct fuse_req *req;
+       struct fuse_file *ff;
+       struct inode *inode;
+};
+
+static void fuse_writepages_send(struct fuse_fill_wb_data *data)
+{
+       struct fuse_req *req = data->req;
+       struct inode *inode = data->inode;
+       struct fuse_conn *fc = get_fuse_conn(inode);
+       struct fuse_inode *fi = get_fuse_inode(inode);
+
+       req->ff = fuse_file_get(data->ff);
+       spin_lock(&fc->lock);
+       list_add_tail(&req->list, &fi->queued_writes);
+       fuse_flush_writepages(inode);
+       spin_unlock(&fc->lock);
+}
+
+static int fuse_writepages_fill(struct page *page,
+               struct writeback_control *wbc, void *_data)
+{
+       struct fuse_fill_wb_data *data = _data;
+       struct fuse_req *req = data->req;
+       struct inode *inode = data->inode;
+       struct fuse_conn *fc = get_fuse_conn(inode);
+       struct page *tmp_page;
+       int err;
+
+       if (!data->ff) {
+               err = -EIO;
+               data->ff = fuse_write_file_get(fc, get_fuse_inode(inode));
+               if (!data->ff)
+                       goto out_unlock;
+       }
+
+       if (req) {
+               BUG_ON(!req->num_pages);
+               if (req->num_pages == FUSE_MAX_PAGES_PER_REQ ||
+                   (req->num_pages + 1) * PAGE_CACHE_SIZE > fc->max_write ||
+                   req->pages[req->num_pages - 1]->index + 1 != page->index) {
+
+                       fuse_writepages_send(data);
+                       data->req = NULL;
+               }
+       }
+       err = -ENOMEM;
+       tmp_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
+       if (!tmp_page)
+               goto out_unlock;
+
+       /*
+        * The page must not be redirtied until the writeout is completed
+        * (i.e. userspace has sent a reply to the write request).  Otherwise
+        * there could be more than one temporary page instance for each real
+        * page.
+        *
+        * This is ensured by holding the page lock in page_mkwrite() while
+        * checking fuse_page_is_writeback().  We already hold the page lock
+        * since clear_page_dirty_for_io() and keep it held until we add the
+        * request to the fi->writepages list and increment req->num_pages.
+        * After this fuse_page_is_writeback() will indicate that the page is
+        * under writeback, so we can release the page lock.
+        */
+       if (data->req == NULL) {
+               struct fuse_inode *fi = get_fuse_inode(inode);
+
+               err = -ENOMEM;
+               req = fuse_request_alloc_nofs(FUSE_MAX_PAGES_PER_REQ);
+               if (!req) {
+                       __free_page(tmp_page);
+                       goto out_unlock;
+               }
+
+               fuse_write_fill(req, data->ff, page_offset(page), 0);
+               req->misc.write.in.write_flags |= FUSE_WRITE_CACHE;
+               req->in.argpages = 1;
+               req->background = 1;
+               req->num_pages = 0;
+               req->end = fuse_writepage_end;
+               req->inode = inode;
+
+               spin_lock(&fc->lock);
+               list_add(&req->writepages_entry, &fi->writepages);
+               spin_unlock(&fc->lock);
+
+               data->req = req;
+       }
+       set_page_writeback(page);
+
+       copy_highpage(tmp_page, page);
+       req->pages[req->num_pages] = tmp_page;
+       req->page_descs[req->num_pages].offset = 0;
+       req->page_descs[req->num_pages].length = PAGE_SIZE;
+
+       inc_bdi_stat(page->mapping->backing_dev_info, BDI_WRITEBACK);
+       inc_zone_page_state(tmp_page, NR_WRITEBACK_TEMP);
+       end_page_writeback(page);
+
+       /*
+        * Protected by fc->lock against concurrent access by
+        * fuse_page_is_writeback().
+        */
+       spin_lock(&fc->lock);
+       req->num_pages++;
+       spin_unlock(&fc->lock);
+
+       err = 0;
+out_unlock:
+       unlock_page(page);
+
+       return err;
+}
+
+static int fuse_writepages(struct address_space *mapping,
+                          struct writeback_control *wbc)
+{
+       struct inode *inode = mapping->host;
+       struct fuse_fill_wb_data data;
+       int err;
+
+       err = -EIO;
+       if (is_bad_inode(inode))
+               goto out;
+
+       data.inode = inode;
+       data.req = NULL;
+       data.ff = NULL;
+
+       err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
+       if (data.req) {
+               /* Ignore errors if we can write at least one page */
+               BUG_ON(!data.req->num_pages);
+               fuse_writepages_send(&data);
+               err = 0;
+       }
+       if (data.ff)
+               fuse_file_put(data.ff, false);
+out:
+       return err;
+}
+
 static int fuse_launder_page(struct page *page)
 {
        int err = 0;
@@ -2607,6 +2750,7 @@ static const struct file_operations fuse_direct_io_file_operations = {
 static const struct address_space_operations fuse_file_aops  = {
        .readpage       = fuse_readpage,
        .writepage      = fuse_writepage,
+       .writepages     = fuse_writepages,
        .launder_page   = fuse_launder_page,
        .readpages      = fuse_readpages,
        .set_page_dirty = __set_page_dirty_nobuffers,