On Wed, 8 Jul 2015, Andrew Morton wrote:
> On Wed, 8 Jul 2015 19:03:08 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>
> >
> >
> > On Tue, 7 Jul 2015, Andrew Morton wrote:
> >
> > > On Tue, 7 Jul 2015 11:10:09 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
> > >
> > > > Introduce the functions kvmalloc and kvmalloc_node. These functions
> > > > provide reliable allocation of object of arbitrary size. They attempt to
> > > > do allocation with kmalloc and if it fails, use vmalloc. Memory allocated
> > > > with these functions should be freed with kvfree.
> > >
> > > Sigh. We've resisted doing this because vmalloc() is somewhat of a bad
> > > thing, and we don't want to make it easy for people to do bad things.
> > >
> > > And vmalloc is bad because a) it's slow and b) it does GFP_KERNEL
> > > allocations for page tables and c) it is susceptible to arena
> > > fragmentation.
> >
> > This patch makes less use of vmalloc.
> >
> > The typical pattern is that someone notices random failures due to memory
> > fragmentation in some subsystem that uses large kmalloc - so he replaces
> > kmalloc with vmalloc - and the code gets slower because of that. With this
> > patch, you can replace many vmalloc users with kvmalloc - and vmalloc will
> > be used only very rarely, when the memory is too fragmented for kmalloc.
>
> Yes, I guess there is that.
>
> > Here I'm sending next version of the patch with comments added.
>
> You didn't like kvzalloc()? We can always add those later...
>
> > --- linux-4.2-rc1.orig/include/linux/mm.h 2015-07-07 15:58:11.000000000 +0200
> > +++ linux-4.2-rc1/include/linux/mm.h 2015-07-08 19:22:24.000000000 +0200
> > @@ -400,6 +400,11 @@ static inline int is_vmalloc_or_module_a
> > }
> > #endif
> >
> > +extern void *kvmalloc_node(size_t size, gfp_t gfp, int node);
> > +static inline void *kvmalloc(size_t size, gfp_t gfp)
> > +{
> > + return kvmalloc_node(size, gfp, NUMA_NO_NODE);
> > +}
> > extern void kvfree(const void *addr);
> >
> > static inline void compound_lock(struct page *page)
> > 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?
Mikulas
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