Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Dec 19, 2022 at 04:01:00PM +0800, Baoquan He wrote:
> On 12/17/22 at 11:44am, Lorenzo Stoakes wrote:
> > On Sat, Dec 17, 2022 at 09:54:30AM +0800, Baoquan He wrote:
> > > @@ -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,
> > >  				    (va->va_end - va->va_start));
> > >  	free_unmap_vmap_area(va);
> >
> > Would it be better to perform the BUG_ON() after the lock is released? You
> > already check if va exists before unmasking so it's safe.
>
> It's a little unclear to me why we care BUG_ON() is performed before or
> after the lock released. We won't have a stable kernel after BUG_ON()(),
> right?

BUG_ON()'s can be recoverable in user context and it would be a very simple
change that would not fundamentally alter anything to simply place the added
lines prior to the BUG_ON().

The code as-is doesn't really make sense anyway, you BUG_ON(!va) then check if
va is non-null, then immediately the function afterwards passes va around as if
it were not null, so I think it'd also be an aesthetic and logical improvement
:)

> >
> > Also, do we want to clear VMAP_BLOCK here?
>
> I do, but I don't find a good place to clear VMAP_BLOCK.
>
> In v1, I tried to clear it in free_vmap_area_noflush() as below,
> Uladzislau dislikes it. So I remove it. My thinking is when we unmap and
> free the vmap area, the vmap_area is moved from vmap_area_root into
> &free_vmap_area_root. When we allocate a new vmap_area via
> alloc_vmap_area(), we will allocate a new va by kmem_cache_alloc_node(),
> the va->flags must be 0. Seems not initializing it to 0 won't impact
> thing.
>

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?

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?

> 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);
>
>         nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
>
> >
>