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Date: Sat, 20 Jan 2024 12:55:10 +0000
From: Lorenzo Stoakes <lstoakes@gmail.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>, Baoquan He <bhe@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Liam R . Howlett" <Liam.Howlett@oracle.com>,
	Dave Chinner <david@fromorbit.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v3 04/11] mm: vmalloc: Remove global vmap_area_root
 rb-tree
Message-ID: <2c318a40-9e0f-4d24-b5cc-e712f7b2c334@lucifer.local>
References: <20240102184633.748113-1-urezki@gmail.com>
 <20240102184633.748113-5-urezki@gmail.com>
 <63104f8e-2fe3-46b2-842c-f11f8bb4b336@lucifer.local>
 <Zakkc5jrxmJj-plk@pc636>
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In-Reply-To: <Zakkc5jrxmJj-plk@pc636>

On Thu, Jan 18, 2024 at 02:15:31PM +0100, Uladzislau Rezki wrote:

[snip]

>
> > > +	struct rb_root root;
> > > +	struct list_head head;
> > > +	spinlock_t lock;
> > > +};
> > > +
> > > +static struct vmap_node {
> > > +	/* Bookkeeping data of this node. */
> > > +	struct rb_list busy;
> > > +} single;
> >
> > This may be a thing about encapsulation/naming or similar, but I'm a little
> > confused as to why the rb_list type is maintained as a field rather than
> > its fields embedded?
> >
> The "struct vmap_node" will be extended by the following patches in the
> series.
>

Yeah sorry I missed this, only realising after I sent...!

> > > +
> > > +static struct vmap_node *vmap_nodes = &single;
> > > +static __read_mostly unsigned int nr_vmap_nodes = 1;
> > > +static __read_mostly unsigned int vmap_zone_size = 1;
> >
> > It might be worth adding a comment here explaining that we're binding to a
> > single node for now to maintain existing behaviour (and a brief description
> > of what these values mean - for instance what unit vmap_zone_size is
> > expressed in?)
> >
> Right. Agree on it :)
>

Indeed :)

[snip]

> > >  /* Look up the first VA which satisfies addr < va_end, NULL if none. */
> > > -static struct vmap_area *find_vmap_area_exceed_addr(unsigned long addr)
> > > +static struct vmap_area *
> > > +find_vmap_area_exceed_addr(unsigned long addr, struct rb_root *root)
> > >  {
> > >  	struct vmap_area *va = NULL;
> > > -	struct rb_node *n = vmap_area_root.rb_node;
> > > +	struct rb_node *n = root->rb_node;
> > >
> > >  	addr = (unsigned long)kasan_reset_tag((void *)addr);
> > >
> > > @@ -1552,12 +1583,14 @@ __alloc_vmap_area(struct rb_root *root, struct list_head *head,
> > >   */
> > >  static void free_vmap_area(struct vmap_area *va)
> > >  {
> > > +	struct vmap_node *vn = addr_to_node(va->va_start);
> > > +
> >
> > I'm being nitty here, and while I know it's a vmalloc convention to use
> > 'va' and 'vm', perhaps we can break away from the super short variable name
> > convention and use 'vnode' or something for these values?
> >
> > I feel people might get confused between 'vm' and 'vn' for instance.
> >
> vnode, varea?

I think 'vm' and 'va' are fine, just scanning through easy to mistake 'vn'
and 'vm'. Obviously a litle nitpicky! You could replace all but a bit
churny, so I think vn -> vnode works best imo.

[snip]

> > >  struct vmap_area *find_vmap_area(unsigned long addr)
> > >  {
> > > +	struct vmap_node *vn;
> > >  	struct vmap_area *va;
> > > +	int i, j;
> > >
> > > -	spin_lock(&vmap_area_lock);
> > > -	va = __find_vmap_area(addr, &vmap_area_root);
> > > -	spin_unlock(&vmap_area_lock);
> > > +	/*
> > > +	 * An addr_to_node_id(addr) converts an address to a node index
> > > +	 * where a VA is located. If VA spans several zones and passed
> > > +	 * addr is not the same as va->va_start, what is not common, we
> > > +	 * may need to scan an extra nodes. See an example:
> >
> > For my understading when you say 'scan an extra nodes' do you mean scan
> > just 1 extra node, or multiple? If the former I'd replace this with 'may
> > need to scan an extra node' if the latter then 'may ened to scan extra
> > nodes'.
> >
> > It's a nitty language thing, but also potentially changes the meaning of
> > this!
> >
> Typo, i should replace it to: scan extra nodes.

Thanks.

>
> > > +	 *
> > > +	 *      <--va-->
> > > +	 * -|-----|-----|-----|-----|-
> > > +	 *     1     2     0     1
> > > +	 *
> > > +	 * VA resides in node 1 whereas it spans 1 and 2. If passed
> > > +	 * addr is within a second node we should do extra work. We
> > > +	 * should mention that it is rare and is a corner case from
> > > +	 * the other hand it has to be covered.
> >
> > A very minor language style nit, but you've already said this is not
> > common, I don't think you need this 'We should mention...' bit. It's not a
> > big deal however!
> >
> No problem. We can remove it!

Thanks.

>
> > > +	 */
> > > +	i = j = addr_to_node_id(addr);
> > > +	do {
> > > +		vn = &vmap_nodes[i];
> > >
> > > -	return va;
> > > +		spin_lock(&vn->busy.lock);
> > > +		va = __find_vmap_area(addr, &vn->busy.root);
> > > +		spin_unlock(&vn->busy.lock);
> > > +
> > > +		if (va)
> > > +			return va;
> > > +	} while ((i = (i + 1) % nr_vmap_nodes) != j);
> >
> > If you comment above suggests that only 1 extra node might need to be
> > scanned, should we stop after one iteration?
> >
> Not really. Though we can improve it further to scan backward.

I think it'd be good to clarify in the comment above that the VA could span
more than 1 node then, as the diagram seems to imply only 1 (I think just
simply because of the example you were showing).

[snip]

> > >  static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > >  {
> > > +	struct vmap_node *vn;
> > >  	struct vmap_area *va;
> > > +	int i, j;
> > >
> > > -	spin_lock(&vmap_area_lock);
> > > -	va = __find_vmap_area(addr, &vmap_area_root);
> > > -	if (va)
> > > -		unlink_va(va, &vmap_area_root);
> > > -	spin_unlock(&vmap_area_lock);
> > > +	i = j = addr_to_node_id(addr);
> > > +	do {
> > > +		vn = &vmap_nodes[i];
> > >
> > > -	return va;
> > > +		spin_lock(&vn->busy.lock);
> > > +		va = __find_vmap_area(addr, &vn->busy.root);
> > > +		if (va)
> > > +			unlink_va(va, &vn->busy.root);
> > > +		spin_unlock(&vn->busy.lock);
> > > +
> > > +		if (va)
> > > +			return va;
> > > +	} while ((i = (i + 1) % nr_vmap_nodes) != j);
> >
> > Maybe worth adding a comment saying to refer to the comment in
> > find_vmap_area() to see why this loop is necessary.
> >
> OK. We can do it to make it better for reading.

Thanks!

[snip]

> > > @@ -3728,8 +3804,11 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
> >
> > Unrelated to your change but makes me feel a little unwell to see 'const
> > char *addr'! Can we change this at some point? Or maybe I can :)
> >
> You are welcome :)

Haha ;) yes I think I might tbh, I have noted it down.

>
> > >
> > >  	remains = count;
> > >
> > > -	spin_lock(&vmap_area_lock);
> > > -	va = find_vmap_area_exceed_addr((unsigned long)addr);
> > > +	/* Hooked to node_0 so far. */
> > > +	vn = addr_to_node(0);
> >
> > Why can't we use addr for this call? We already enforce the node-0 only
> > thing by setting nr_vmap_nodes to 1 right? And won't this be potentially
> > subtly wrong when we later increase this?
> >
> I used to have 0 here. But please note, it is changed by the next patch in
> this series.

Yeah sorry, again hadn't noticed this.

[snip]

> > > +		spin_lock(&vn->busy.lock);
> > > +		insert_vmap_area(vas[area], &vn->busy.root, &vn->busy.head);
> > >  		setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
> > >  				 pcpu_get_vm_areas);
> > > +		spin_unlock(&vn->busy.lock);
> >
> > Hmm, before we were locking/unlocking once before the loop, now we're
> > locking on each iteration, this seems inefficient.
> >
> > Seems like we need logic like:
> >
> > /* ... something to check nr_vms > 0 ... */
> > struct vmap_node *last_node = NULL;
> >
> > for (...) {
> > 	struct vmap_node *vnode = addr_to_node(vas[area]->va_start);
> >
> > 	if (vnode != last_node) {
> > 		spin_unlock(last_node->busy.lock);
> > 		spin_lock(vnode->busy.lock);
> > 		last_node = vnode;
> > 	}
> >
> > 	...
> > }
> >
> > if (last_node)
> > 	spin_unlock(last_node->busy.lock);
> >
> > To minimise the lock twiddling. What do you think?
> >
> This per-cpu-allocator prefetches several VA units per-cpu. I do not
> find it as critical because it is not a hot path for the per-cpu allocator.
> When its buffers are exhausted it does an extra prefetch. So it is not
> frequent.

OK, sure I mean this is simpler and more readable so if not a huge perf
concern then not a big deal.

>
> >
> > >  	}
> > > -	spin_unlock(&vmap_area_lock);
> > >
> > >  	/*
> > >  	 * Mark allocated areas as accessible. Do it now as a best-effort
> > > @@ -4253,55 +4333,57 @@ bool vmalloc_dump_obj(void *object)
> > >  {
> > >  	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > >  	const void *caller;
> > > -	struct vm_struct *vm;
> > >  	struct vmap_area *va;
> > > +	struct vmap_node *vn;
> > >  	unsigned long addr;
> > >  	unsigned int nr_pages;
> > > +	bool success = false;
> > >
> > > -	if (!spin_trylock(&vmap_area_lock))
> > > -		return false;
> >
> > Nitpick on style for this, I really don't know why you are removing this
> > early exit? It's far neater to have a guard clause than to nest a whole
> > bunch of code below.
> >
> Hm... I can return back as it used to be. I do not have a strong opinion here.

Yeah that'd be ideal just for readability.

[snip the rest as broadly fairly trivial comment stuff on which we agree]

>
> Thank you for the review! I can fix the comments as separate patches if
> no objections.

Yes, overall it's style/comment improvement stuff nothing major, feel free
to send as follow-up patches.

I don't want to hold anything up here so for the rest, feel free to add:

Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>

>
> --
> Uladzislau Rezki