> >
> > A few questions about the resulting alloc_vmap_area():
> >
> > : static struct vmap_area *alloc_vmap_area(unsigned long size,
> > : unsigned long align,
> > : unsigned long vstart, unsigned long vend,
> > : int node, gfp_t gfp_mask)
> > : {
> > : struct vmap_area *va, *pva;
> > : unsigned long addr;
> > : int purged = 0;
> > :
> > : BUG_ON(!size);
> > : BUG_ON(offset_in_page(size));
> > : BUG_ON(!is_power_of_2(align));
> > :
> > : if (unlikely(!vmap_initialized))
> > : return ERR_PTR(-EBUSY);
> > :
> > : might_sleep();
> > :
> > : va = kmem_cache_alloc_node(vmap_area_cachep,
> > : gfp_mask & GFP_RECLAIM_MASK, node);
> >
> > Why does this use GFP_RECLAIM_MASK? Please add a comment explaining
> > this.
> >
I need to think about it. Initially it was like that.
> > : if (unlikely(!va))
> > : return ERR_PTR(-ENOMEM);
> > :
> > : /*
> > : * Only scan the relevant parts containing pointers to other objects
> > : * to avoid false negatives.
> > : */
> > : kmemleak_scan_area(&va->rb_node, SIZE_MAX, gfp_mask & GFP_RECLAIM_MASK);
> > :
> > : retry:
> > : /*
> > : * Preload this CPU with one extra vmap_area object. It is used
> > : * when fit type of free area is NE_FIT_TYPE. Please note, it
> > : * does not guarantee that an allocation occurs on a CPU that
> > : * is preloaded, instead we minimize the case when it is not.
> > : * It can happen because of migration, because there is a race
> > : * until the below spinlock is taken.
> > : *
> > : * The preload is done in non-atomic context, thus it allows us
> > : * to use more permissive allocation masks to be more stable under
> > : * low memory condition and high memory pressure.
> > : *
> > : * Even if it fails we do not really care about that. Just proceed
> > : * as it is. "overflow" path will refill the cache we allocate from.
> > : */
> > : if (!this_cpu_read(ne_fit_preload_node)) {
> >
> > Readability nit: local `pva' should be defined here, rather than having
> > function-wide scope.
> >
> > : pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
> >
> > Why doesn't this honour gfp_mask? If it's not a bug, please add
> > comment explaining this.
> >
But there is a comment, if understand you correctly:
<snip>
* Even if it fails we do not really care about that. Just proceed
* as it is. "overflow" path will refill the cache we allocate from.
<snip>
> > The kmem_cache_alloc() in adjust_va_to_fit_type() omits the caller's
> > gfp_mask also. If not a bug, please document the unexpected behaviour.
> >
I will add a comment there.
>
> These questions appear to be for the code that this patch touches, not
> for the patch itself.
>
> > :
> > : if (this_cpu_cmpxchg(ne_fit_preload_node, NULL,
> > pva)) { : if (pva)
> > : kmem_cache_free(vmap_area_cachep,
> > pva); : }
> > : }
> > :
> > : spin_lock(&vmap_area_lock);
> > :
> > : /*
> > : * If an allocation fails, the "vend" address is
> > : * returned. Therefore trigger the overflow path.
> > : */
> >
> > As for the intent of this patch, why not preallocate the vmap_area
> > outside the spinlock and use it within the spinlock? Does spin_lock()
> > disable preemption on RT? I forget, but it doesn't matter much anyway
>
> spin_lock() does not disable preemption on RT. But it does disable
> migration (thus the task should remain on the current CPU).
>
> > - doing this will make the code better in the regular kernel I think?
> > Something like this:
> >
> > struct vmap_area *pva = NULL;
> >
> > ...
> >
> > if (!this_cpu_read(ne_fit_preload_node))
> > pva = kmem_cache_alloc_node(vmap_area_cachep, ...);
> >
> > spin_lock(&vmap_area_lock);
> >
> > if (pva && __this_cpu_cmpxchg(ne_fit_preload_node, NULL, pva))
> > kmem_cache_free(vmap_area_cachep, pva);
> >
>
>
> This looks fine to me.
>
Yes, i agree that is better. I was thinking about doing so, but decided
to keep as it is, because of low number of "corner cases" anyway.
I will upload the v2.
Thanks for the comments!
--
Vlad Rezki
