Re: [PATCH 2/5] fuse: writepages: crop secondary requests on send -v2

[Miklos Szeredi]

On Wed, Oct 2, 2013 at 1:17 PM, Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx> wrote:
> If writeback happens while fuse is in FUSE_NOWRITE condition, the request
> will be queued but not processed immediately (see fuse_flush_writepages()).
> Until FUSE_NOWRITE becomes relaxed, more writebacks can happen. They will
> be queued as "secondary" requests to that first ("primary") request.
>
> When FUSE_NOWRITE is relaxed and fuse_send_writepage() is called, it must
> crop both primary and secondary requests according to the actual i_size.
> Otherwise, if only primary is cropped, an extending write(2) may increase
> i_size soon and then secondary requests won't be cropped properly. The result
> would be stale data written to the server to a file offset where zeros must be.
>
> Changed in v2:
>  - avoid NULL pointer dereference in fuse_drop_writepage().
>
> Signed-off-by: Maxim Patlasov <MPatlasov@xxxxxxxxxxxxx>
> ---
>  fs/fuse/file.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 55 insertions(+), 9 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 575e44f..89a2e76 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1435,6 +1435,51 @@ static void fuse_writepage_finish(struct fuse_conn *fc, struct fuse_req *req)
>         wake_up(&fi->page_waitq);
>  }
>
> +/* Drop list of secondary writepage requests */
> +static void fuse_drop_writepage(struct fuse_conn *fc, struct fuse_req *req)
> +{
> +       struct backing_dev_info *bdi = req ?
> +               req->inode->i_mapping->backing_dev_info : NULL;
> +
> +       while (req) {
> +               struct fuse_req *next = req->misc.write.next;
> +               dec_bdi_stat(bdi, BDI_WRITEBACK);
> +               dec_zone_page_state(req->pages[0], NR_WRITEBACK_TEMP);
> +               fuse_writepage_free(fc, req);
> +               fuse_put_request(fc, req);
> +               req = next;
> +       }
> +}
> +
> +/* Crop the misc.write.in.size of parent and secondary writepage requests */
> +static bool fuse_crop_writepage(struct fuse_conn *fc, struct fuse_req *req,
> +                               loff_t size, struct fuse_req **drop_list)
> +{
> +       if (req->misc.write.in.offset >= size)
> +               return true;
> +
> +       while (req) {
> +               struct fuse_req *next = req->misc.write.next;
> +               struct fuse_write_in *inarg = &req->misc.write.in;
> +               __u64 data_size = inarg->size ? :
> +                       req->num_pages * PAGE_CACHE_SIZE;
> +
> +               if (inarg->offset + data_size <= size) {
> +                       inarg->size = data_size;
> +               } else if (inarg->offset < size) {
> +                       inarg->size = size - inarg->offset;
> +               } else {
> +                       /* Got truncated off completely */
> +                       req->misc.write.next = *drop_list;
> +                       *drop_list = req;

This corrupts the list (the req is not taken off the list before being
added to another).  It could be fixed, but why not check this instead
in fuse_writepage_end() before queuing the next request?

Thanks,
Miklos


> +               }
> +
> +               req = next;
> +       }
> +
> +       return false;
> +}
> +
>  /* Called under fc->lock, may release and reacquire it */
>  static void fuse_send_writepage(struct fuse_conn *fc, struct fuse_req *req)
>  __releases(fc->lock)
> @@ -1443,29 +1488,30 @@ __acquires(fc->lock)
>         struct fuse_inode *fi = get_fuse_inode(req->inode);
>         loff_t size = i_size_read(req->inode);
>         struct fuse_write_in *inarg = &req->misc.write.in;
> -       __u64 data_size = req->num_pages * PAGE_CACHE_SIZE;
> +       struct fuse_req *drop_list = NULL;
>
>         if (!fc->connected)
>                 goto out_free;
>
> -       if (inarg->offset + data_size <= size) {
> -               inarg->size = data_size;
> -       } else if (inarg->offset < size) {
> -               inarg->size = size - inarg->offset;
> -       } else {
> -               /* Got truncated off completely */
> -               goto out_free;
> -       }
> +       if (fuse_crop_writepage(fc, req, size, &drop_list))
> +               goto out_free; /* drop req and descendants */
>
>         req->in.args[1].size = inarg->size;
>         fi->writectr++;
>         fuse_request_send_background_locked(fc, req);
> +
> +       if (drop_list) {
> +               spin_unlock(&fc->lock);
> +               fuse_drop_writepage(fc, drop_list);
> +               spin_lock(&fc->lock);
> +       }
>         return;
>
>   out_free:
>         fuse_writepage_finish(fc, req);
>         spin_unlock(&fc->lock);
>         fuse_writepage_free(fc, req);
> +       fuse_drop_writepage(fc, req->misc.write.next);
>         fuse_put_request(fc, req);
>         spin_lock(&fc->lock);
>  }
>
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