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

[Michal Hocko <mhocko@kernel.org>]

On Tue 03-01-17 11:23:04, Vlastimil Babka wrote:
> On 01/02/2017 02:37 PM, Michal Hocko wrote:
> > 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.
> > 
> > Changes since v1
> > - define __vmalloc_node_flags for CONFIG_MMU=n
> > 
> > Cc: Anatoly Stepanov <astepanov@cloudlinux.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Mike Snitzer <snitzer@redhat.com>
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: "Theodore Ts'o" <tytso@mit.edu>
> > Reviewed-by: Andreas Dilger <adilger@dilger.ca> # ext4 part
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> (but with a small fix and suggestion below)

Thanks!

> 
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -346,6 +346,46 @@ unsigned long vm_mmap(struct file *file, unsigned long addr,
> >  }
> >  EXPORT_SYMBOL(vm_mmap);
> > 
> > +/**
> > + * kvmalloc_node - allocate contiguous memory from SLAB with vmalloc fallback
> > + * @size: size of the request.
> > + * @flags: gfp mask for the allocation - must be compatible with GFP_KERNEL.
> > + * @node: numa node to allocate from
> > + *
> > + * Uses kmalloc to get the memory but if the allocation fails then falls back
> > + * to the vmalloc allocator. Use kvfree for freeing the memory.
> > + */
> > +void *kvmalloc_node(size_t size, gfp_t flags, int node)
> > +{
> > +	gfp_t kmalloc_flags = flags;
> > +	void *ret;
> > +
> > +	/*
> > +	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
> > +	 * so the given set of flags has to be compatible.
> > +	 */
> > +	WARN_ON((flags & GFP_KERNEL) != GFP_KERNEL);
> 
> Wouldn't a _ONCE be sufficient? It's unlikely that multiple wrong call sites
> appear out of the blue, but we don't want to flood the log from a single
> frequently called site. No strong feelings though.

Fair enough, I will make it WARN_ON_ONCE. I wish WARN_ON_ONCE would be
more clever, though. We can lose information about different call sites.
I was thinking about how to deal with it and I stackdepot sounds like it
could help here. But this is off-topic...

> > +
> > +	/*
> > +	 * Make sure that larger requests are not too disruptive - no OOM
> > +	 * killer and no allocation failure warnings as we have a fallback
> > +	 */
> > +	if (size > PAGE_SIZE)
> > +		kmalloc_flags |= __GFP_NORETRY | __GFP_NOWARN;
> > +
> > +	ret = kmalloc_node(size, kmalloc_flags, node);
> > +
> > +	/*
> > +	 * It doesn't really make sense to fallback to vmalloc for sub page
> > +	 * requests
> > +	 */
> > +	if (ret || size < PAGE_SIZE)
> 
> This should be size <= PAGE_SIZE.

You are right of course!
-- 
Michal Hocko
SUSE Labs