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SubjectRe: [PATCH 1/1] mm/vmalloc: remove preempt_disable/enable when do preloading
On Wed,  9 Oct 2019 18:49:34 +0200 "Uladzislau Rezki (Sony)" <urezki@gmail.com> wrote:

> Get rid of preempt_disable() and preempt_enable() when the
> preload is done for splitting purpose. The reason is that
> calling spin_lock() with disabled preemtion is forbidden in
> CONFIG_PREEMPT_RT kernel.
>
> Therefore, we do not guarantee that a CPU is preloaded, instead
> we minimize the case when it is not with this change.
>
> For example i run the special test case that follows the preload
> pattern and path. 20 "unbind" threads run it and each does
> 1000000 allocations. Only 3.5 times among 1000000 a CPU was
> not preloaded thus. So it can happen but the number is rather
> negligible.
>
> ...
>

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.

: 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.

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.

:
: 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
- 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);




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