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Date:   Fri, 13 Jan 2023 11:55:07 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        stephen.s.brennan@oracle.com, willy@infradead.org,
        akpm@linux-foundation.org, hch@infradead.org
Subject: Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area
Message-ID: <Y8DWG+OHV6E4cR8p@fedora>
References: <20221217015435.73889-1-bhe@redhat.com>
 <20221217015435.73889-3-bhe@redhat.com>
 <Y6HpGayyQZH7U7Fd@pc636>
 <Y6UrPGMVYUMttKD3@MiWiFi-R3L-srv>
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In-Reply-To: <Y6UrPGMVYUMttKD3@MiWiFi-R3L-srv>
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Hi Uladzislau Rezki，

On 12/23/22 at 12:14pm, Baoquan He wrote:
> On 12/20/22 at 05:55pm, Uladzislau Rezki wrote:
......
 > >  	spin_lock(&vmap_area_lock);
> > >  	insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
> > > @@ -1887,6 +1889,10 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> > >  
> > >  #define VMAP_BLOCK_SIZE		(VMAP_BBMAP_BITS * PAGE_SIZE)
> > >  
> > > +#define VMAP_RAM		0x1
> > > +#define VMAP_BLOCK		0x2
> > > +#define VMAP_FLAGS_MASK		0x3
> > > 
> > Maybe to rename a VMAP_BLOCK to something like VMAP_BLOCK_RESERVED or
> > VMAP_PER_CPU_BLOCK?
> 
> Both VMAP_BLOCK or VMAP_PER_CPU_BLOCK look good to me, please see my
> explanation at below.
> 
> > 
> > >  struct vmap_block_queue {
> > >  	spinlock_t lock;
> > >  	struct list_head free;
> > > @@ -1962,7 +1968,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> > >  
> > >  	va = alloc_vmap_area(VMAP_BLOCK_SIZE, VMAP_BLOCK_SIZE,
> > >  					VMALLOC_START, VMALLOC_END,
> > > -					node, gfp_mask);
> > > +					node, gfp_mask,
> > > +					VMAP_RAM|VMAP_BLOCK);
> > >
> > A new_vmap_block() is for a per-cpu path. As far as i see the VMAP_BLOCK
> > flag is used to mark a VA that corresponds to a reserved per-cpu free area.
> > 
> > Whereas a VMAP_RAM is for VA that was obtained over per-cpu path but
> > over alloc_vmap_area() thus a VA should be read out over "busy" tree
> > directly.

Rethinking about the vmap->flags and the bit0->VMAP_RAM,
bit1->VMAP_BLOCK correspondence, it looks better to use bit0->VMAP_RAM
to indicate the vm_map_ram area, no matter how it's handled inside
vm_map_ram() interface; and use bit1->VMAP_BLOCK to mark out the special
vm_map_ram area which is further subdivided and managed by struct
vmap_block. With these, you can see that we can identify vm_map_ram area
and treat it as one type of vmalloc area, e.g in vread(), s_show().

Means when we are talking about vm_map_ram areas, we use
(vmap->flags & VMAP_RAM) to recognize them; when we need to
differentiate and handle vm_map_ram areas respectively, we use
(vmap->flags & VMAP_BLOCK) to pick out the area which is further managed
by vmap_block. Please help check if this is OK to you.

> > 
> > Why do you need to set here both VMAP_RAM and VMAP_BLOCK?
> 
> My understanding is that the vm_map_ram area has two types, one is
> the vb percpu area via vb_alloc(), the other is allocated via
> alloc_vmap_area(). While both of them is got from vm_map_ram()
> interface, this is the main point that distinguishes the vm_map_ram area
> than the normal vmalloc area, and this makes vm_map_ram area not owning
> va->vm pointer. So here, I use flag VMAP_RAM to mark the vm_map_ram
> area, including the two types; meanwhile, I add VMAP_BLOCK to mark out
> the vb percpu area. 
> 
> I understand people could have different view about them, e.g as you
> said, use VMAP_RAM to mark the type of vm_map_ram area allocated through
> alloc_vmap_area(), while use VMAP_PER_CPU_BLOCK to mark vb percpu area
> from vb_alloc. In this way, we may need to rename VMAP_RAM to reflect
> the area allocated from alloc_vmap_area() only. Both is fine to me.
> 
> > 
> > >  	if (IS_ERR(va)) {
> > >  		kfree(vb);
> > >  		return ERR_CAST(va);
> > > @@ -2229,8 +2236,12 @@ void vm_unmap_ram(const void *mem, unsigned int count)
> > >  		return;
> > >  	}
> > >  
> > > -	va = find_vmap_area(addr);
> > > +	spin_lock(&vmap_area_lock);
> > > +	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> > >  	BUG_ON(!va);
> > > +	if (va)
> > > +		va->flags &= ~VMAP_RAM;
> > > +	spin_unlock(&vmap_area_lock);
> > >  	debug_check_no_locks_freed((void *)va->va_start,
> > >
> > Agree with Lorenzo. BUG_ON() should be out of spinlock(). Furthermore
> > i think it makes sense to go with WARN_ON_ONCE() and do not kill a system.
> > Instead emit a warning and bailout.
> > 
> > What do you think? Maybe separate patch for it?
> 
> Agree, your patch looks great to me. Thanks.
> 
> > 
> > >  				    (va->va_end - va->va_start));
> > >  	free_unmap_vmap_area(va);
> > > @@ -2265,7 +2276,8 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
> > >  	} else {
> > >  		struct vmap_area *va;
> > >  		va = alloc_vmap_area(size, PAGE_SIZE,
> > > -				VMALLOC_START, VMALLOC_END, node, GFP_KERNEL);
> > > +				VMALLOC_START, VMALLOC_END,
> > > +				node, GFP_KERNEL, VMAP_RAM);
> > >  		if (IS_ERR(va))
> > >  			return NULL;
> > >  
> > > @@ -2505,7 +2517,7 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
> > >  	if (!(flags & VM_NO_GUARD))
> > >  		size += PAGE_SIZE;
> > >  
> > > -	va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
> > > +	va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0);
> > >  	if (IS_ERR(va)) {
> > >  		kfree(area);
> > >  		return NULL;
> > >
> > I know we have already discussed the new parameter. But what if we just
> > use atomic_set operation to mark VA as either vmap-ram or vmap-block?

As I replied at above, I take the vm_map_ram as one kind of vmalloc
area, and mark out the percpu vmap block handling of vm_map_ram area.
Seems the passing in the flags through function parameter is better. Not
sure if I got your suggestion correctly, and my code change is
appropriate. I have sent v3 according to your and Lorenzo's comments and
suggestion, and my rethinking after reading your words. I make some
adjustment to try to remove misundersanding or confusion when reading
patch and code. Please help check if it's OK.