Re: [PATCH 2/2] mm: drop PF_MEMALLOC_NORECLAIM

Vlastimil Babka
2024-08-29T21:45:38Z

On 8/26/24 21:18, Michal Hocko wrote:
> On Mon 26-08-24 18:49:23, Matthew Wilcox wrote:
>> On Mon, Aug 26, 2024 at 06:51:55PM +0200, Michal Hocko wrote:
> [...]
>> > If a plan revert is preferably, I will go with it.
>>
>> There aren't any other users of PF_MEMALLOC_NOWARN and it definitely
>> seems like something you want at a callsite rather than blanket for every
>> allocation below this point. We don't seem to have many PF_ flags left,
>> so let's not keep it around if there's no immediate plans for it.
>
> Good point. What about this?
> ---
> From 923cd429d4b1a3520c93bcf46611ae74a3158865 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Mon, 26 Aug 2024 21:15:02 +0200
> Subject: [PATCH] Revert "mm: introduce PF_MEMALLOC_NORECLAIM,
> PF_MEMALLOC_NOWARN"
>
> This reverts commit eab0af905bfc3e9c05da2ca163d76a1513159aa4.
>
> There is no existing user of those flags. PF_MEMALLOC_NOWARN is
> dangerous because a nested allocation context can use GFP_NOFAIL which
> could cause unexpected failure. Such a code would be hard to maintain
> because it could be deeper in the call chain.
>
> PF_MEMALLOC_NORECLAIM has been added even when it was pointed out [1]
> that such a allocation contex is inherently unsafe if the context
> doesn't fully control all allocations called from this context.
>
> While PF_MEMALLOC_NOWARN is not dangerous the way PF_MEMALLOC_NORECLAIM
> is it doesn't have any user and as Matthew has pointed out we are
> running out of those flags so better reclaim it without any real users.
>
> [1] https://lore.kernel.org/all/ZcM0xtlKbAOFjv5n@tiehlicka/
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> include/linux/sched.h | 4 ++--
> include/linux/sched/mm.h | 17 ++++-------------
> 2 files changed, 6 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f8d150343d42..731ff1078c9e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1657,8 +1657,8 @@ extern struct pid *cad_pid;
> * I am cleaning dirty pages from some other bdi. */
> #define PF_KTHREAD 0x00200000 /* I am a kernel thread */
> #define PF_RANDOMIZE 0x00400000 /* Randomize virtual address space */
> -#define PF_MEMALLOC_NORECLAIM 0x00800000 /* All allocation requests will clear __GFP_DIRECT_RECLAIM */
> -#define PF_MEMALLOC_NOWARN 0x01000000 /* All allocation requests will inherit __GFP_NOWARN */
> +#define PF__HOLE__00800000 0x00800000
> +#define PF__HOLE__01000000 0x01000000
> #define PF__HOLE__02000000 0x02000000
> #define PF_NO_SETAFFINITY 0x04000000 /* Userland is not allowed to meddle with cpus_mask */
> #define PF_MCE_EARLY 0x08000000 /* Early kill for mce process policy */
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 91546493c43d..07c4fde32827 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -258,25 +258,16 @@ static inline gfp_t current_gfp_context(gfp_t flags)
> {
> unsigned int pflags = READ_ONCE(current->flags);
>
> - if (unlikely(pflags & (PF_MEMALLOC_NOIO |
> - PF_MEMALLOC_NOFS |
> - PF_MEMALLOC_NORECLAIM |
> - PF_MEMALLOC_NOWARN |
> - PF_MEMALLOC_PIN))) {
> + if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) {
> /*
> - * Stronger flags before weaker flags:
> - * NORECLAIM implies NOIO, which in turn implies NOFS
> + * NOIO implies both NOIO and NOFS and it is a weaker context
> + * so always make sure it makes precedence
> */
> - if (pflags & PF_MEMALLOC_NORECLAIM)
> - flags &= ~__GFP_DIRECT_RECLAIM;
> - else if (pflags & PF_MEMALLOC_NOIO)
> + if (pflags & PF_MEMALLOC_NOIO)
> flags &= ~(__GFP_IO | __GFP_FS);
> else if (pflags & PF_MEMALLOC_NOFS)
> flags &= ~__GFP_FS;
>
> - if (pflags & PF_MEMALLOC_NOWARN)
> - flags |= __GFP_NOWARN;
> -
> if (pflags & PF_MEMALLOC_PIN)
> flags &= ~__GFP_MOVABLE;
> }