Re: [RFC v2 4/4] fs: Optimize credentials reference count for backing file ops

[Amir Goldstein]

On Fri, Jan 26, 2024 at 1:57 AM Vinicius Costa Gomes
<vinicius.gomes@xxxxxxxxx> wrote:
>
> For backing file operations, users are expected to pass credentials
> that will outlive the backing file common operations.
>
> Use the specialized guard statements to override/revert the
> credentials.
>

As I wrote before, I prefer to see this patch gets reviewed and merged
before the overlayfs large patch, so please reorder the series.

> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
> ---
>  fs/backing-file.c | 124 ++++++++++++++++++++++------------------------
>  1 file changed, 60 insertions(+), 64 deletions(-)
>
> diff --git a/fs/backing-file.c b/fs/backing-file.c
> index a681f38d84d8..9874f09f860f 100644
> --- a/fs/backing-file.c
> +++ b/fs/backing-file.c
> @@ -140,7 +140,7 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
>                                struct backing_file_ctx *ctx)
>  {
>         struct backing_aio *aio = NULL;
> -       const struct cred *old_cred;
> +       const struct cred *old_cred = ctx->cred;
>         ssize_t ret;
>
>         if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)))
> @@ -153,29 +153,28 @@ ssize_t backing_file_read_iter(struct file *file, struct iov_iter *iter,
>             !(file->f_mode & FMODE_CAN_ODIRECT))
>                 return -EINVAL;
>
> -       old_cred = override_creds(ctx->cred);
> -       if (is_sync_kiocb(iocb)) {
> -               rwf_t rwf = iocb_to_rw_flags(flags);
> +       scoped_guard(cred, old_cred) {

This reads very strage.

Also, I see that e.g. scoped_guard(spinlock_irqsave, ... hides the local var
used for save/restore of flags inside the macro.

Perhaps you use the same technique for scoped_guard(cred, ..
loose the local old_cred variable in all those functions and then the
code will read:

scoped_guard(cred, ctx->cred) {

which is nicer IMO.

> +               if (is_sync_kiocb(iocb)) {
> +                       rwf_t rwf = iocb_to_rw_flags(flags);
>
> -               ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
> -       } else {
> -               ret = -ENOMEM;
> -               aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
> -               if (!aio)
> -                       goto out;
> +                       ret = vfs_iter_read(file, iter, &iocb->ki_pos, rwf);
> +               } else {
> +                       ret = -ENOMEM;
> +                       aio = kmem_cache_zalloc(backing_aio_cachep, GFP_KERNEL);
> +                       if (!aio)
> +                               goto out;
>
> -               aio->orig_iocb = iocb;
> -               kiocb_clone(&aio->iocb, iocb, get_file(file));
> -               aio->iocb.ki_complete = backing_aio_rw_complete;
> -               refcount_set(&aio->ref, 2);
> -               ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
> -               backing_aio_put(aio);
> -               if (ret != -EIOCBQUEUED)
> -                       backing_aio_cleanup(aio, ret);
> +                       aio->orig_iocb = iocb;
> +                       kiocb_clone(&aio->iocb, iocb, get_file(file));
> +                       aio->iocb.ki_complete = backing_aio_rw_complete;
> +                       refcount_set(&aio->ref, 2);
> +                       ret = vfs_iocb_iter_read(file, &aio->iocb, iter);
> +                       backing_aio_put(aio);
> +                       if (ret != -EIOCBQUEUED)
> +                               backing_aio_cleanup(aio, ret);
> +               }

if possible, I would rather avoid all this churn in functions that mostly
do work with the new cred, so either use guard(cred, ) directly or split a
helper that uses guard(cred, ) form the rest.

Thanks,
Amir.