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

From: Dave Chinner
Date: Thu Dec 08 2016 - 20:45:19 EST

On Thu, Dec 08, 2016 at 11:33:00AM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 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.

See fs/xfs/kmem.c::kmem_zalloc_large(), which is XFS's version of
kvmalloc() that is GFP_NOFS/GFP_NOIO safe. Any generic API for this
functionality will have to play these memalloc_noio_save/
memalloc_noio_restore games to ensure they are GFP_NOFS safe....


Dave Chinner