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

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/27/21 10:43 PM, Noah Goldstein wrote:
> On Mon, May 24, 2021 at 7:51 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>> -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?

Far from being a fast path, so would rather keep it simpler
and cleaner

> 
> 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.

-- 
Pavel Begunkov