linux-kernel - Re: [PATCH v1 2/7] mm/vmalloc.c: add flags to mark vm_map

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Date:   Mon, 5 Dec 2022 13:56:29 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stephen.s.brennan@...cle.com, urezki@...il.com,
        willy@...radead.org, akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH v1 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area

> Through vmalloc API, a virtual kernel area is reserved for physical
> address mapping. And vmap_area is used to track them, while vm_struct
> is allocated to associate with the vmap_area to store more information
> and passed out.
> 
> However, area reserved via vm_map_ram() is an exception. It doesn't have
> vm_struct to associate with vmap_area. And we can't recognize the
> vmap_area with '->vm == NULL' as a vm_map_ram() area because the normal
> freeing path will set va->vm = NULL before unmapping, please see
> function remove_vm_area().
> 
> Meanwhile, there are two types of vm_map_ram area. One is the whole
> vmap_area being reserved and mapped at one time; the other is the
> whole vmap_area with VMAP_BLOCK_SIZE size being reserved, while mapped
> into split regions with smaller size several times via vb_alloc().
> 
> To mark the area reserved through vm_map_ram(), add flags field into
> struct vmap_area. Bit 0 indicates whether it's a vm_map_ram area,
> while bit 1 indicates whether it's a vmap_block type of vm_map_ram
> area.
> 
> This is a preparatoin for later use.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  include/linux/vmalloc.h |  1 +
>  mm/vmalloc.c            | 18 +++++++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 096d48aa3437..69250efa03d1 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -76,6 +76,7 @@ struct vmap_area {
>  		unsigned long subtree_max_size; /* in "free" tree */
>  		struct vm_struct *vm;           /* in "busy" tree */
>  	};
> +	unsigned long flags; /* mark type of vm_map_ram area */
>  };
>  
>  /* archs that select HAVE_ARCH_HUGE_VMAP should override one or more of these */
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 5d3fd3e6fe09..d6f376060d83 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1815,6 +1815,7 @@ static void free_vmap_area_noflush(struct vmap_area *va)
>  
>  	spin_lock(&vmap_area_lock);
>  	unlink_va(va, &vmap_area_root);
> +	va->flags = 0;
>  	spin_unlock(&vmap_area_lock);
>  
This is not a good place to set flags to zero. It looks to me like
corner and kind of specific.


>  	nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
> @@ -1887,6 +1888,10 @@ 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
> +
>  struct vmap_block_queue {
>  	spinlock_t lock;
>  	struct list_head free;
> @@ -1967,6 +1972,9 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  		kfree(vb);
>  		return ERR_CAST(va);
>  	}
> +	spin_lock(&vmap_area_lock);
> +	va->flags = VMAP_RAM|VMAP_BLOCK;
> +	spin_unlock(&vmap_area_lock);
>
The per-cpu code was created as a fast per-cpu allocator because of high
vmalloc lock contention. If possible we should avoid of locking of the
vmap_area_lock. Because it has a high contention.

>  
>  	vaddr = vmap_block_vaddr(va->va_start, 0);
>  	spin_lock_init(&vb->lock);
> @@ -2229,8 +2237,12 @@ 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,
>  				    (va->va_end - va->va_start));
>  	free_unmap_vmap_area(va);
> @@ -2269,6 +2281,10 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
>  		if (IS_ERR(va))
>  			return NULL;
>  
> +		spin_lock(&vmap_area_lock);
> +		va->flags = VMAP_RAM;
> +		spin_unlock(&vmap_area_lock);
> +
>
Same here.

--
Uladzislau Rezki