On Fri, May 29, 2020 at 02:46:48PM +0200, Hannes Reinecke wrote:
> On 5/29/20 1:21 PM, Matthew Wilcox wrote:
> > On Fri, May 29, 2020 at 08:50:21AM +0200, Hannes Reinecke wrote:
> > > > I meant just use xa_alloc() for everything instead of using xa_insert for
> > > > 0-255.
> > > >
> > > But then I'll have to use xa_find() to get to the actual element as the 1:1
> > > mapping between SCSI LUN and array index is lost.
> > > And seeing that most storage arrays will expose only up to 256 LUNs I
> > > thought this was a good improvement on lookup.
> > > Of course, this only makes sense if xa_load() is more efficient than
> > > xa_find(). If not then of course it's a bit futile.
> >
> > xa_load() is absolutely more efficient than xa_find(). It's just a
> > question of whether it matters ;-) Carry on ...
> >
> Thanks. I will.
>
> BTW, care to update the documentation?
>
> * Return: 0 on success, -ENOMEM if memory could not be allocated or
> * -EBUSY if there are no free entries in @limit.
> */
> int __xa_alloc(struct xarray *xa, u32 *id, void *entry,
> struct xa_limit limit, gfp_t gfp)
> {
> XA_STATE(xas, xa, 0);
>
> if (WARN_ON_ONCE(xa_is_advanced(entry)))
> return -EINVAL;
> if (WARN_ON_ONCE(!xa_track_free(xa)))
> return -EINVAL;
>
> So looks as if the documentation is in need of updating to cover -EINVAL.
> And, please, state somewhere that whenever you want to use xa_alloc() you
> _need_ to specify XA_ALLOC_FLAGS in xa_init_flags(), otherwise
> you trip across the above.
Like this?
Allocating XArrays
------------------
If you use DEFINE_XARRAY_ALLOC() to define the XArray, or
initialise it by passing ``XA_FLAGS_ALLOC`` to xa_init_flags(),
the XArray changes to track whether entries are in use or not.
You can call xa_alloc() to store the entry at an unused index
in the XArray. If you need to modify the array from interrupt context,
you can use xa_alloc_bh() or xa_alloc_irq() to disable
interrupts while allocating the ID.
> It's not entirely obvious, and took me the better half of a day to figure
> out.
Really? You get a nice WARN_ON and backtrace so you can see where
you went wrong ... What could I change to make this easier?
