Re: [PATCH RFC 5/5] non-mm: discourage the usage of __GFP_NOFAIL and encourage GFP_NOFAIL

[Barry Song <21cnbao@gmail.com>]

On Thu, Jul 25, 2024 at 12:17 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 24-07-24 20:55:44, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > GFP_NOFAIL includes the meaning of block and direct reclamation, which
> > is essential for a true no-fail allocation. We are gradually starting
> > to enforce this block semantics to prevent the potential misuse of
> > __GFP_NOFAIL in atomic contexts in the future.
> >
> > A typical example of incorrect usage is in VDPA, where GFP_ATOMIC
> > and __GFP_NOFAIL are used together.
>
> Ohh, so you have done the migration. Please squash those two patches.
> Also if we want to preserve clean __GFP_NOFAIL for internal MM use then it
> should be moved away from include/linux/gfp_types.h. But is there any
> real use for that?

yes. currently i got two,

lib/rhashtable.c

static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
                                               size_t nbuckets,
                                               gfp_t gfp)
{
        struct bucket_table *tbl = NULL;
        size_t size;
        int i;
        static struct lock_class_key __key;

        tbl = alloc_hooks_tag(ht->alloc_tag,
                        kvmalloc_node_noprof(struct_size(tbl, buckets,
nbuckets),
                                             gfp|__GFP_ZERO, NUMA_NO_NODE));

        size = nbuckets;

        if (tbl == NULL && (gfp & ~__GFP_NOFAIL) != GFP_KERNEL) {
                tbl = nested_bucket_table_alloc(ht, nbuckets, gfp);
                nbuckets = 0;
        }

        ...

        return tbl;
}

and tools/perf/builtin-kmem.c:

static const struct {
        const char *original;
        const char *compact;
} gfp_compact_table[] = {
        { "GFP_TRANSHUGE",              "THP" },
        { "GFP_TRANSHUGE_LIGHT",        "THL" },
        { "GFP_HIGHUSER_MOVABLE",       "HUM" },
        { "GFP_HIGHUSER",               "HU" },
        ...
        { "__GFP_NOFAIL",               "NF" },
       ...
};

>
> --
> Michal Hocko
> SUSE Labs