On Mon, 26 Aug 2024, 陈玉凡 wrote:
> 在 2024/8/24 3:01, Mikulas Patocka 写道:
>
>
> On Fri, 23 Aug 2024, 陈玉凡 wrote:
>
> 在 2024/8/23 1:08, Mikulas Patocka 写道:
>
> On Thu, 22 Aug 2024, Chen Yufan wrote:
>
> Use time_after_eq macro instead of opening it for readability.
>
> Signed-off-by: Chen Yufan <chenyufan@xxxxxxxx>
> ---
> drivers/md/dm-writecache.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> index 7ce8847b3..548d4d37e 100644
> --- a/drivers/md/dm-writecache.c
> +++ b/drivers/md/dm-writecache.c
> @@ -16,6 +16,7 @@
> #include <linux/pfn_t.h>
> #include <linux/libnvdimm.h>
> #include <linux/delay.h>
> +#include <linux/jiffies.h>
> #include "dm-io-tracker.h"
>
> #define DM_MSG_PREFIX "writecache"
> @@ -1994,8 +1995,8 @@ static void writecache_writeback(struct work_struct *work)
> while (!list_empty(&wc->lru) &&
> (wc->writeback_all ||
> wc->freelist_size + wc->writeback_size <= wc->freelist_low_watermark ||
> - (jiffies - container_of(wc->lru.prev, struct wc_entry, lru)->age >=
> - wc->max_age - wc->max_age / MAX_AGE_DIV))) {
> + (time_after_eq(jiffies, container_of(wc->lru.prev, struct wc_entry, lru)->age +
> + (wc->max_age - wc->max_age / MAX_AGE_DIV))))) {
>
> n_walked++;
> if (unlikely(n_walked > WRITEBACK_LATENCY) &&
> --
> 2.39.0
>
> I'm not sure about this. The old and new code is not really equivalent.
>
> Mikulas
>
> The code here is susceptible to overflow issues, and the time_*() macros
> can handle this.
>
> Chen
>
> So, describe some case (i.e. the values of jiffies,
> container_of(wc->lru.prev, struct wc_entry, lru)->age and wc->max_age)
> where the old code misbehaves and the new code doesn't.
>
> If we want to fix a bug, we need to know what the bug actually is.
>
> Mikulas
>
> When jiffies increased beyond the maximum value of unsigned long, it
> wraps around to zero, and the value of jiffies would be smaller than the
> container_of(wc->lru.prev, struct wc_entry, lru)->age value despite
> logically being larger. Eventurally, because of the wraparound, the
> result of the condition would be wrong.
>
> Chen
For example, if "jiffies" is 0x10 (because it wrapped around) and
"container_of(wc->lru.prev, struct wc_entry, lru)->age" is 0xfffffff0,
then the expression "jiffies - container_of(wc->lru.prev, struct wc_entry,
lru)->age" would be 0x20. That is the correct value, I don't see any
problem with this.
Mikulas
