linux-kernel - Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map

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Date:   Mon, 19 Dec 2022 13:01:43 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, urezki@...il.com,
        stephen.s.brennan@...cle.com, willy@...radead.org,
        akpm@...ux-foundation.org, hch@...radead.org
Subject: Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area

On Mon, Dec 19, 2022 at 08:24:47PM +0800, Baoquan He wrote:
> In fact, I should not do the checking, but do the clearing anyway. If I
> change it as below, does it look better to you?
>
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 5e578563784a..369b848d097a 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2253,8 +2253,7 @@ void vm_unmap_ram(const void *mem, unsigned int count)
>  	spin_lock(&vmap_area_lock);
>  	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
>  	BUG_ON(!va);
> -	if (va)
> -		va->flags &= ~VMAP_RAM;
> +	va->flags &= ~VMAP_RAM;
>  	spin_unlock(&vmap_area_lock);
>  	debug_check_no_locks_freed((void *)va->va_start,
>  				    (va->va_end - va->va_start));

This is better as it avoids the slightly contradictory situation of checking for
a condition we've asserted is not the case, but I would still far prefer keeping
this as-is and placing the unlock before the BUG_ON().

This avoids explicitly and knowingly holding a lock over a BUG_ON() and is
simple to implement, e.g.:-

  	spin_lock(&vmap_area_lock);
  	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
 	if (va)
 		va->flags &= ~VMAP_RAM;
  	spin_unlock(&vmap_area_lock);
  	BUG_ON(!va);

> > You are at this point clearing the VMAP_RAM flag though, so if it is unimportant
> > what the flags are after this call, why are you clearing this one?
>
> With my understanding, We had better do the clearing. Currently, from
> the code, not doing the clearing won't cause issue. If possible, I would
> like to clear it as below draft code.
>

Sure, it seems appropriate to clear it, I'm just unsure as to why you aren't
just clearing both flags? Perhaps just set va->flags = 0?

> >
> > It is just a little confusing, I wonder whether the VMAP_BLOCK flag is necessary
> > at all, is it possible to just treat a non-VMAP_BLOCK VMAP_RAM area as if it
> > were simply a fully occupied block? Do we gain much by the distinction?
>
> Yeah, VMAP_BLOCK flag is necessary. vmap_block contains used region,
> or dirty/free regions. While the non-vmap_blcok vm_map_ram area is
> similar with the non vm_map_ram area. When reading out vm_map_ram
> regions, vmap_block regions need be treated differently.

OK looking through again closely I see you're absolutely right, I wondered
whether you could somehow make a non-VMAP_BLOCK vread() operation be equivalent
to a block one (only across the whole mapping), but I don't think you can.