linux-kernel - Re: [PATCH 1/1] mm/vmalloc: remove preempt

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Date:   Thu, 10 Oct 2019 17:17:49 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Daniel Wagner <dwagner@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] mm/vmalloc: remove preempt_disable/enable when do
 preloading

> > 
> > 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