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@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.
> >
> > 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@xxxxxxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> > Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> > Cc: "Theodore Ts'o" <tytso@xxxxxxx>
> > Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx> # ext4 part
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> (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
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