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SubjectRe: [RFC][PATCH] flexible array implementation
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On Tue, 2009-07-21 at 13:18 -0700, Andrew Morton wrote:
> On Tue, 21 Jul 2009 09:03:33 -0700
> Dave Hansen <dave@linux.vnet.ibm.com> wrote:
> > Once a structure goes over PAGE_SIZE*2, we see occasional
> > allocation failures. Some people have chosen to switch
> > over to things like vmalloc() that will let them keep
> > array-like access to such a large structures. But,
> > vmalloc() has plenty of downsides.
>
> Thanks for looking into this.
>
> > Here's an alternative. I think it's what Andrew was
> > suggesting here:
> >
> > http://lkml.org/lkml/2009/7/2/518
> >
> > I call it a flexible array. It does all of its work in
> > PAGE_SIZE bits, so never does an order>0 allocation.
> > The base level has PAGE_SIZE-2*sizeof(int) bytes of
> > storage for pointers to the second level. So, with a
> > 32-bit arch, you get about 4MB (4183112 bytes) of total
> > storage when the objects pack nicely into a page. It
> > is half that on 64-bit because the pointers are twice
> > the size.
> >
> > The interface is dirt simple. 4 functions:
> > alloc_flex_array()
> > free_flex_array()
>
> flex_array_alloc() and flex_array_free(), please.

Will do.

> The interface is rather unexpected. Some callers will want
> random-access writes and will have sparse data sets. Can we make it
> more array-like?

I changed the flex_array_put() function to take an index and added a
flex_array_append() that just calls flex_array_put() along with the
->nr_elements variable manipulations.

> What are the constraints of this implementation? Maximum index, maximum
> number of elements, etc?
>
> What are the locking rules? Caller-provided, obviously (and
> correctly). If the caller wants to use a spinlock the caller must use
> GFP_ATOMIC and handle the fallout when that fails. (But they'd need to
> handle the fallout with mutex/GFP_KERNEL too).

I've added some comments in the kerneldoc for the (newly renamed) alloc
function:

* The maximum number of elements is currently the number of elements
* that can be stored in a page times the number of page pointers
* that we can fit in the base structure or (using integer math):
*
* (PAGE_SIZE/element_size) * (PAGE_SIZE-8)/sizeof(void *)
*
* Here's a table showing example capacities. Note that the maximum
* index that the get/put() functions is just nr_objects-1.
*
* Element size | Objects | Objects |
* PAGE_SIZE=4k | 32-bit | 64-bit |
* ----------------------------------|
* 1 byte | 4186112 | 2093056 |
* 2 bytes | 2093056 | 1046528 |
* 3 bytes | 1395030 | 697515 |
* 4 bytes | 1046528 | 523264 |
* 32 bytes | 130816 | 65408 |
* 33 bytes | 126728 | 63364 |
* 2048 bytes | 2044 | 10228 |
* 2049 bytes | 1022 | 511 |
* void * | 1046528 | 261632 |
*
* Since 64-bit pointers are twice the size, we lose half the
* capacity in the base structure. Also note that no effort is made
* to efficiently pack objects across page boundaries.
I figure it's less likely to get lost there than in the patch
description.

> > We could also potentially just pass the "element_size"
> > into each of the API functions instead of storing it
> > internally. That would get us one more base pointer
> > on 32-bit.
> >
> > The last improvement that I thought about was letting
> > the individual array members span pages. In this
> > implementation, if you have a 2049-byte object, it
> > will only pack one of them into each "part" with
> > no attempt to pack them. At this point, I don't think
> > the added complexity would be worth it.
>
> I expect the majority of callers will be storing plain old pointers in here.
>
> In fact the facility would be quite useful if it explicitly stored and
> returned void*'s, like radix-tree and IDR.

I think that would be just as easy as sticking some helpers around it

int flex_array_put_ptr(struct flex_array *fa, int element_nr,
void *ptr, gfp_t flags)
{
return flex_array_put(fa, element_nr, &ptr, flags);
}
void *flex_array_get_ptr(struct flex_array *fa, int element_nr)
{
return *(void **)flex_array_get(fa, element_nr);
}
Or, as you say, we may not want the store-by-value semantics at all. In
that case, we can just do this by default.

> Do we know of any potential callers which would want flex_array to
> store elements by value in this manner?

The original user I was thinking of was the one that spawned this idea,
the pid_t:

> if (PIDLIST_REALLOC_DIFFERENCE(length, dest)) {
> - newlist = kmalloc(dest * sizeof(pid_t), GFP_KERNEL);
> + newlist = pidlist_allocate(dest);
> if (newlist) {
> memcpy(newlist, list, dest * sizeof(pid_t));
> - kfree(list);
> + pidlist_free(list);
> *p = newlist;
> }

I think they wanted elements by value.

> > +struct flex_array *alloc_flex_array(int element_size, int total, gfp_t flags)
> > +{
> > + struct flex_array *ret;
> > + int max_size = __nr_part_ptrs() * __elements_per_part(element_size);
> > +
> > + /* max_size will end up 0 if element_size > PAGE_SIZE */
> > + if (total > max_size)
> > + return NULL;
> > + ret = kzalloc(sizeof(struct flex_array), flags);
> > + if (!ret)
> > + return NULL;
> > + ret->element_size = element_size;
> > + return ret;
> > +}
>
> I expect that a decent proportion of users of this facility will only
> ever want a single flex_array. So they'll want to be able to define and
> initialise their flex_array at compile-time.
>
> That looks pretty easy to do?

Initializing the base structure should be pretty easy. I've included a
FLEX_ARRAY_INIT() function to do it.

New patch on the way...

-- Dave



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