Re: [PATCH 10/13] io_uring: add helpers for 2 level table alloc

[Noah Goldstein <goldstein.w.n@xxxxxxxxx>]

On Mon, May 24, 2021 at 7:51 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>
> Some parts like fixed file table use 2 level tables, factor out helpers
> for allocating/deallocating them as more users are to come.
>
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> ---
>  fs/io_uring.c | 73 ++++++++++++++++++++++++++++++---------------------
>  1 file changed, 43 insertions(+), 30 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 40b70c34c1b2..1cc2d16637ff 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -7054,14 +7054,36 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>         return READ_ONCE(rings->cq.head) == READ_ONCE(rings->cq.tail) ? ret : 0;
>  }
>
> -static void io_free_file_tables(struct io_file_table *table, unsigned nr_files)
> +static void io_free_page_table(void **table, size_t size)
>  {
> -       unsigned i, nr_tables = DIV_ROUND_UP(nr_files, IORING_MAX_FILES_TABLE);
> +       unsigned i, nr_tables = DIV_ROUND_UP(size, PAGE_SIZE);
>
>         for (i = 0; i < nr_tables; i++)
> -               kfree(table->files[i]);
> -       kfree(table->files);
> -       table->files = NULL;
> +               kfree(table[i]);
> +       kfree(table);
> +}
> +
> +static void **io_alloc_page_table(size_t size)
> +{
> +       unsigned i, nr_tables = DIV_ROUND_UP(size, PAGE_SIZE);
> +       size_t init_size = size;
> +       void **table;
> +
> +       table = kcalloc(nr_tables, sizeof(*table), GFP_KERNEL);
> +       if (!table)
> +               return NULL;
> +
> +       for (i = 0; i < nr_tables; i++) {
> +               unsigned int this_size = min(size, PAGE_SIZE);
> +
> +               table[i] = kzalloc(this_size, GFP_KERNEL);
> +               if (!table[i]) {
> +                       io_free_page_table(table, init_size);
> +                       return NULL;
Unless zalloc returns non-NULL for size == 0, you are guranteed to do
this for size <= PAGE_SIZE * (nr_tables - 1).  Possibly worth calculating early?

If you calculate early you could then make the loop:

for (i = 0; i < nr_tables - 1; i++) {
    table[i] = kzalloc(PAGE_SIZE, GFP_KERNEL);
    if (!table[i]) {
        io_free_page_table(table, init_size);
        return NULL;
    }
}

table[i] = kzalloc(size - (nr_tables - 1) * PAGE_SIZE, GFP_KERNEL);
    if (!table[i]) {
        io_free_page_table(table, init_size);
        return NULL;
    }

Which is almost certainly faster.

> +               }
> +               size -= this_size;
> +       }
> +       return table;
>  }
>
>  static inline void io_rsrc_ref_lock(struct io_ring_ctx *ctx)
> @@ -7190,6 +7212,22 @@ static int io_rsrc_data_alloc(struct io_ring_ctx *ctx, rsrc_put_fn *do_put,
>         return 0;
>  }
>
> +static bool io_alloc_file_tables(struct io_file_table *table, unsigned nr_files)
> +{
> +       size_t size = nr_files * sizeof(struct io_fixed_file);
> +
> +       table->files = (struct io_fixed_file **)io_alloc_page_table(size);
> +       return !!table->files;
> +}
> +
> +static void io_free_file_tables(struct io_file_table *table, unsigned nr_files)
> +{
> +       size_t size = nr_files * sizeof(struct io_fixed_file);
> +
> +       io_free_page_table((void **)table->files, size);
> +       table->files = NULL;
> +}
> +
>  static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
>  {
>  #if defined(CONFIG_UNIX)
> @@ -7451,31 +7489,6 @@ static int io_sqe_files_scm(struct io_ring_ctx *ctx)
>  }
>  #endif
>
> -static bool io_alloc_file_tables(struct io_file_table *table, unsigned nr_files)
> -{
> -       unsigned i, nr_tables = DIV_ROUND_UP(nr_files, IORING_MAX_FILES_TABLE);
> -
> -       table->files = kcalloc(nr_tables, sizeof(*table->files), GFP_KERNEL);
> -       if (!table->files)
> -               return false;
> -
> -       for (i = 0; i < nr_tables; i++) {
> -               unsigned int this_files = min(nr_files, IORING_MAX_FILES_TABLE);
> -
> -               table->files[i] = kcalloc(this_files, sizeof(*table->files[i]),
> -                                       GFP_KERNEL);
> -               if (!table->files[i])
> -                       break;
> -               nr_files -= this_files;
> -       }
> -
> -       if (i == nr_tables)
> -               return true;
> -
> -       io_free_file_tables(table, nr_tables * IORING_MAX_FILES_TABLE);
> -       return false;
> -}
> -
>  static void io_rsrc_file_put(struct io_ring_ctx *ctx, struct io_rsrc_put *prsrc)
>  {
>         struct file *file = prsrc->file;
> --
> 2.31.1
>