On Tue, 14 Jul 2015, Mike Snitzer wrote:
> > > > > Index: linux-4.2-rc1/mm/util.c
> > > > > ===================================================================
> > > > > --- linux-4.2-rc1.orig/mm/util.c 2015-07-07 15:58:11.000000000 +0200
> > > > > +++ linux-4.2-rc1/mm/util.c 2015-07-08 19:22:26.000000000 +0200
> > > > > @@ -316,6 +316,61 @@ unsigned long vm_mmap(struct file *file,
> > > > > }
> > > > > EXPORT_SYMBOL(vm_mmap);
> > > > >
> > > > > +void *kvmalloc_node(size_t size, gfp_t gfp, int node)
> > > > > +{
> > > > > + void *p;
> > > > > + unsigned uninitialized_var(noio_flag);
> > > > > +
> > > > > + /* vmalloc doesn't support no-wait allocations */
> > > > > + WARN_ON_ONCE(!(gfp & __GFP_WAIT));
> > > > > +
> > > > > + if (likely(size <= KMALLOC_MAX_SIZE)) {
> > > > > + /*
> > > > > + * Use __GFP_NORETRY so that we don't loop waiting for the
> > > > > + * allocation - we don't have to loop here, if the memory
> > > > > + * is too fragmented, we fallback to vmalloc.
> > > >
> > > > I'm not sure about this decision. The direct reclaim retry code is the
> > > > normal default behaviour and becomes more important with larger allocation
> > > > attempts. So why turn it off, and make it more likely that we return
> > > > vmalloc memory?
> > >
> > > It can avoid triggering the OOM killer in case of fragmented memory.
> > >
> > > This is general question - if the code can handle allocation failure
> > > gracefully, what gfp flags should it use? Maybe add some flag
> > > __GFP_MAYFAIL instead of __GFP_NORETRY that changes the behavior in
> > > desired way?
> > >
> >
> > There's a misunderstanding in regards to the comment: __GFP_NORETRY
> > doesn't turn direct reclaim or compaction off, it is still attempted and
> > with the same priority as any other allocation. This only stops the page
> > allocator from calling the oom killer, which will free memory or panic the
> > system, and looping when memory is available.
> >
> > In regards to the proposal in general, I think it's unnecessary because we
> > are still left behind with other users who open code their call to
> > vmalloc. I was interested in commit 058504edd026 ("fs/seq_file: fallback
> > to vmalloc allocation") since it solved an issue with high memory
> > fragmentation. Note how it falls back to vmalloc(): _without_ this
> > __GFP_NORETRY. That's because we only want to fallback when high-order
> > allocations fail and the page allocator doesn't implicitly loop due to the
> > order. ext4_kvmalloc(), ext4_kzmalloc() does the same.
> >
> > The differences in implementations between those that do kmalloc() and
> > fallback to vmalloc() are different enough that I don't think we need this
> > addition.
>
> Wouldn't mm benefit from acknowledging the pattern people are
> open-coding and switching existing code over to official methods for
> accomplishing the same?
>
Sure, but it's not accomplishing the same thing: things like
ext4_kvmalloc() only want to fallback to vmalloc() when high-order
allocations fail: the function is used for different sizes. This cannot
be converted to kvmalloc_node() since it fallsback immediately when
reclaim fails. Same issue with single_file_open() for the seq_file code.
We could go through every kmalloc() -> vmalloc() fallback for more
examples in the code, but those two instances were the first I looked at
and couldn't be converted to kvmalloc_node() without work.
> It is always easier to shoehorn utility functions locally within a
> subsystem (be it ext4, dm, etc) but once enough do something in a
> similar but different way it really should get elevated.
>
I would argue that
void *ext4_kvmalloc(size_t size, gfp_t flags)
{
void *ret;
ret = kmalloc(size, flags | __GFP_NOWARN);
if (!ret)
ret = __vmalloc(size, flags, PAGE_KERNEL);
return ret;
}
is simple enough that we don't need to convert it to anything.
If all such fallback was done in the same way as the implementation as
kvmalloc_node(), and perhaps only very few exceptions were needed, this
would be helpful. Unfortunately, that isn't the case.
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