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From: Uladzislau Rezki <urezki@gmail.com>
Date: Mon, 22 Jan 2024 18:44:34 +0100
To: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>, 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: <Za6pgt2j1n4PTcGI@pc636>
References: <20240102184633.748113-1-urezki@gmail.com>
 <20240102184633.748113-5-urezki@gmail.com>
 <63104f8e-2fe3-46b2-842c-f11f8bb4b336@lucifer.local>
 <Zakkc5jrxmJj-plk@pc636>
 <2c318a40-9e0f-4d24-b5cc-e712f7b2c334@lucifer.local>
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In-Reply-To: <2c318a40-9e0f-4d24-b5cc-e712f7b2c334@lucifer.local>

On Sat, Jan 20, 2024 at 12:55:10PM +0000, Lorenzo Stoakes wrote:
> 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>
> 
Appreciate! I will go through again and send out the patch that adds
more detailed explanation as requested in this review.

Again, thank you!

--
Uladzislau Rezki