On Mon, Jul 24, 2017 at 02:50:50PM +0200, David Sterba wrote:
> On Fri, Jul 21, 2017 at 01:29:08PM -0400, jo...@toxicpanda.com wrote:
> > From: Josef Bacik <jba...@fb.com>
> >
> > Readdir does dir_emit while under the btree lock. dir_emit can trigger
> > the page fault which means we can deadlock. Fix this by allocating a
> > buffer on opening a directory and copying the readdir into this buffer
> > and doing dir_emit from outside of the tree lock.
> >
> > Signed-off-by: Josef Bacik <jba...@fb.com>
> > ---
> > fs/btrfs/inode.c | 110
> > +++++++++++++++++++++++++++++++++++++++++--------------
> > 1 file changed, 83 insertions(+), 27 deletions(-)
> >
> > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> > index 9a4413a..61396e3 100644
> > --- a/fs/btrfs/inode.c
> > +++ b/fs/btrfs/inode.c
> > @@ -5877,6 +5877,56 @@ unsigned char btrfs_filetype_table[] = {
> > DT_UNKNOWN, DT_REG, DT_DIR, DT_CHR, DT_BLK, DT_FIFO, DT_SOCK, DT_LNK
> > };
> >
> > +/*
> > + * All this infrastructure exists because dir_emit can fault, and we are
> > holding
> > + * the tree lock when doing readdir. For now just allocate a buffer and
> > copy
> > + * our information into that, and then dir_emit from the buffer. This is
> > + * similar to what NFS does, only we don't keep the buffer around in
> > pagecache
> > + * because I'm afraid I'll fuck that up.
Can you please explain the concern in more detail?
> > Long term we need to make filldir do
> > + * copy_to_user_inatomic so we don't have to worry about page faulting
> > under the
> > + * tree lock.
> > + */
> > +static int btrfs_opendir(struct inode *inode, struct file *file)
> > +{
> > + struct page *page;
> > +
> > + page = alloc_page(GFP_KERNEL);
> > + if (!page)
> > + return -ENOMEM;
> > + file->private_data = page;
> > + return 0;
> > +}
> > +
> > +static int btrfs_closedir(struct inode *inode, struct file *file)
> > +{
> > + if (file->private_data) {
> > + __free_page((struct page *)file->private_data);
> > + file->private_data = NULL;
> > + }
> > + return 0;
> > +}
> > +
> > +struct dir_entry {
> > + u64 ino;
> > + u64 offset;
> > + unsigned type;
> > + int name_len;
> > +};
> > +
> > +static int btrfs_filldir(void *addr, int entries, struct dir_context *ctx)
> > +{
> > + while (entries--) {
> > + struct dir_entry *entry = addr;
> > + char *name = (char *)(entry + 1);
> > + ctx->pos = entry->offset;
> > + if (!dir_emit(ctx, name, entry->name_len, entry->ino,
> > + entry->type))
> > + return 1;
> > + ctx->pos++;
> > + }
> > + return 0;
> > +}
> > +
> > static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
> > {
> > struct inode *inode = file_inode(file);
> > @@ -5886,16 +5936,17 @@ static int btrfs_real_readdir(struct file *file,
> > struct dir_context *ctx)
> > struct btrfs_key key;
> > struct btrfs_key found_key;
> > struct btrfs_path *path;
> > + struct page *page = file->private_data;
> > + void *addr, *start_addr;
> > struct list_head ins_list;
> > struct list_head del_list;
> > int ret;
> > struct extent_buffer *leaf;
> > int slot;
> > - unsigned char d_type;
> > - int over = 0;
> > - char tmp_name[32];
> > char *name_ptr;
> > int name_len;
> > + int entries = 0;
> > + int total_len = 0;
> > bool put = false;
> > struct btrfs_key location;
> >
> > @@ -5906,6 +5957,7 @@ static int btrfs_real_readdir(struct file *file,
> > struct dir_context *ctx)
> > if (!path)
> > return -ENOMEM;
> >
> > + start_addr = addr = kmap(page);
> > path->reada = READA_FORWARD;
> >
> > INIT_LIST_HEAD(&ins_list);
> > @@ -5921,6 +5973,7 @@ static int btrfs_real_readdir(struct file *file,
> > struct dir_context *ctx)
> > goto err;
> >
> > while (1) {
> > + struct dir_entry *entry;
> > leaf = path->nodes[0];
> > slot = path->slots[0];
> > if (slot >= btrfs_header_nritems(leaf)) {
> > @@ -5942,41 +5995,42 @@ static int btrfs_real_readdir(struct file *file,
> > struct dir_context *ctx)
> > goto next;
> > if (btrfs_should_delete_dir_index(&del_list, found_key.offset))
> > goto next;
> > -
> > - ctx->pos = found_key.offset;
> > -
> > di = btrfs_item_ptr(leaf, slot, struct btrfs_dir_item);
> > if (verify_dir_item(fs_info, leaf, slot, di))
> > goto next;
> >
> > name_len = btrfs_dir_name_len(leaf, di);
> > - if (name_len <= sizeof(tmp_name)) {
> > - name_ptr = tmp_name;
> > - } else {
> > - name_ptr = kmalloc(name_len, GFP_KERNEL);
> > - if (!name_ptr) {
> > - ret = -ENOMEM;
> > - goto err;
> > - }
> > + if ((total_len + sizeof(struct dir_entry) + name_len) >=
> > + PAGE_SIZE) {
>
> How does this work for filenames that of PATH_MAX length? Regardless of
> total_len size, the rest will always overflow PAGE_SIZE if it's 4k.
Ah no, it's filename, so the buffer will be always sufficient. So the
overall approach looks ok to me.
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