Re: [PATCH 2/6] mm: Add become_kswapd and restore_kswapd

Michal Hocko
2020-06-25T12:31:48Z

On Thu 25-06-20 12:31:18, Matthew Wilcox wrote:
> Since XFS needs to pretend to be kswapd in some of its worker threads,
> create methods to save & restore kswapd state. Don't bother restoring
> kswapd state in kswapd -- the only time we reach this code is when we're
> exiting and the task_struct is about to be destroyed anyway.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Certainly better than an opencoded PF_$FOO manipulation
Acked-by: Michal Hocko <mhocko@suse.com>

I would just ask for a clarification because this is rellying to have a
good MM knowledge to follow

> +/*
> + * Tell the memory management that we're a "memory allocator",

I would go with.
Tell the memory management that the caller is working on behalf of the
background memory reclaim (aka kswapd) and help it to make a forward
progress. That means that it will get an access to memory reserves
should there be a need to allocate memory in order to make a forward
progress. Note that the caller has to be extremely careful when doing
that.

Or something like that.

> + * and that if we need more memory we should get access to it
> + * regardless (see "__alloc_pages()"). "kswapd" should
> + * never get caught in the normal page freeing logic.
> + *
> + * (Kswapd normally doesn't need memory anyway, but sometimes
> + * you need a small amount of memory in order to be able to
> + * page out something else, and this flag essentially protects
> + * us from recursively trying to free more memory as we're
> + * trying to free the first piece of memory in the first place).
> + */
> +#define KSWAPD_PF_FLAGS (PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD)
> +
> +static inline unsigned long become_kswapd(void)
> +{
> + unsigned long flags = current->flags & KSWAPD_PF_FLAGS;
> + current->flags |= KSWAPD_PF_FLAGS;
> + return flags;
> +}
> +
> +static inline void restore_kswapd(unsigned long flags)
> +{
> + current->flags &= ~(flags ^ KSWAPD_PF_FLAGS);
> +}
> +
> static inline void set_current_io_flusher(void)
> {
> current->flags |= PF_LOCAL_THROTTLE;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b6d84326bdf2..27ae76699899 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3870,19 +3870,7 @@ static int kswapd(void *p)
> if (!cpumask_empty(cpumask))
> set_cpus_allowed_ptr(tsk, cpumask);
>
> - /*
> - * Tell the memory management that we're a "memory allocator",
> - * and that if we need more memory we should get access to it
> - * regardless (see "__alloc_pages()"). "kswapd" should
> - * never get caught in the normal page freeing logic.
> - *
> - * (Kswapd normally doesn't need memory anyway, but sometimes
> - * you need a small amount of memory in order to be able to
> - * page out something else, and this flag essentially protects
> - * us from recursively trying to free more memory as we're
> - * trying to free the first piece of memory in the first place).
> - */
> - tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
> + become_kswapd();
> set_freezable();
>
> WRITE_ONCE(pgdat->kswapd_order, 0);
> @@ -3932,8 +3920,6 @@ static int kswapd(void *p)
> goto kswapd_try_sleep;
> }
>
> - tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
> -
> return 0;
> }
>
> --
> 2.27.0
>

--
Michal Hocko
SUSE Labs