On Mon, 24 Oct 2022 12:05:56 -0700 Stefan Roesch <shr@xxxxxxxxxxxx> wrote:
> This introduces the bdi_set_max_bytes() function. The max_bytes function
> does not store the max_bytes value. Instead it converts the max_bytes
> value into the corresponding ratio value.
>
> ...
>
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -108,6 +108,7 @@ static inline unsigned long wb_stat_error(void)
> unsigned long long bdi_get_max_bytes(struct backing_dev_info *bdi);
> int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio);
> int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio);
> +int bdi_set_max_bytes(struct backing_dev_info *bdi, unsigned long long max_bytes);
> int bdi_set_strict_limit(struct backing_dev_info *bdi, unsigned int strict_limit);
>
> /*
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 8b8936603783..21d7c1880ea8 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -13,6 +13,7 @@
> */
>
> #include <linux/kernel.h>
> +#include <linux/math64.h>
> #include <linux/export.h>
> #include <linux/spinlock.h>
> #include <linux/fs.h>
> @@ -650,6 +651,28 @@ void wb_domain_exit(struct wb_domain *dom)
> */
> static unsigned int bdi_min_ratio;
>
> +static int bdi_check_pages_limit(unsigned long pages)
> +{
> + unsigned long max_dirty_pages = global_dirtyable_memory();
> +
> + if (pages > max_dirty_pages / 2)
> + return -EINVAL;
> +
> + return 0;
> +}
Some code comments are needed here. Explain what it does and why it
does it. The "/ 2" seems utterly arbitray - explain why this value was
chosen? Why is it better than "/ 3"?
> +static unsigned long bdi_ratio_from_pages(unsigned long pages)
> +{
> + unsigned long background_thresh;
> + unsigned long dirty_thresh;
> + unsigned long ratio;
> +
> + global_dirty_limits(&background_thresh, &dirty_thresh);
> + ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
`unsigned long' is 32-bit on 32-bit machines, which makes this code a
bit odd. Should everything here be u64?
