Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

Michal Hocko
2016-12-08T13:51:46Z

On Thu 08-12-16 14:00:20, David Hildenbrand wrote:
> Am 08.12.2016 um 11:33 schrieb Michal Hocko:
> > From: Michal Hocko <mhocko@suse.com>
> >
> > Using kmalloc with the vmalloc fallback for larger allocations is a
> > common pattern in the kernel code. Yet we do not have any common helper
> > for that and so users have invented their own helpers. Some of them are
> > really creative when doing so. Let's just add kv[mz]alloc and make sure
> > it is implemented properly. This implementation makes sure to not make
> > a large memory pressure for > PAGE_SZE requests (__GFP_NORETRY) and also
> > to not warn about allocation failures. This also rules out the OOM
> > killer as the vmalloc is a more approapriate fallback than a disruptive
> > user visible action.
> >
> > This patch also changes some existing users and removes helpers which
> > are specific for them. In some cases this is not possible (e.g.
> > ext4_kvmalloc, libcfs_kvzalloc, __aa_kvmalloc) because those seems to be
> > broken and require GFP_NO{FS,IO} context which is not vmalloc compatible
> > in general (note that the page table allocation is GFP_KERNEL). Those
> > need to be fixed separately.
> >
> > apparmor has already claimed kv[mz]alloc so remove those and use
> > __aa_kvmalloc instead to prevent from the naming clashes.
> >
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Mike Snitzer <snitzer@redhat.com>
> > Cc: dm-devel@redhat.com
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: "Theodore Ts'o" <tytso@mit.edu>
> > Cc: kvm@vger.kernel.org
> > Cc: linux-ext4@vger.kernel.org
> > Cc: linux-f2fs-devel@lists.sourceforge.net
> > Cc: linux-security-module@vger.kernel.org
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
>
> I remember yet another similar user in arch/s390/kvm/kvm-s390.c
> -> kvm_s390_set_skeys()
>
> ...
> keys = kmalloc_array(args->count, sizeof(uint8_t),
> GFP_KERNEL | __GFP_NOWARN);
> if (!keys)
> vmalloc(sizeof(uint8_t) * args->count);
> ...
>
> would kvmalloc_array make sense? (it would even make the code here
> less error prone and better to read)

Well, if there are more users like that then why not. I just do not want
to duplicate the whole kmalloc API right now. The above could be
trivially changed to kvmalloc(args->count * sizeof(uint8_t), GFP_KERNEL)
so a special API might not be really needed.