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

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Date:   Tue, 20 Dec 2022 12:42:09 +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 Tue, Dec 20, 2022 at 08:14:15PM +0800, Baoquan He wrote:
> Hmm, for the two kinds of vm_map_ram areas, their code paths are
> different. for unmapping vmap_block vm_map_ram, it goes through
> vb_free(). I can only do the clearing in free_vmap_block().
>
>   -->vm_unmap_ram()
>      -->vb_free()
>         -->free_vmap_block()
>
> For non vmap_block vm_map_ram area, I can do the clearing in
> vm_unmap_ram().
>   -->vm_unmap_ram()
>      -->__find_vmap_area()
>      -->free_unmap_vmap_area()
>
> As said earlier, clearing va->flags when unmap vm_map_ram area, or
> clearing va->vm in remove_vm_area(), these have better be done.
> However, not clearing them won't cause issue currently. Because the
> old vmap_area is inserted into free_vmap_area_root, when we allocate a
> new vmap_area through alloc_vmap_area(), we will get q new vmap_area
> from vmap_area_cachep, the old va->flags or va->vm won't be carried into
> the new vmap_area. Clearing them is a good to have, just in case.
>

Sure, this is more so about avoiding confusion and perhaps some future change
which might not take into account that flag state could be invalid.

I guess logically speaking, this is still a block when you are unmapping ram, so
it's not unreasonable to retain the VMAP_BLOCK flag. In that case I'd say we're
good simply clearing VMAP_RAM here. Thanks for the explanations!

> Rethinking about this, I may need to do the clearing when freeing
> vmap_block. Otherwise, people will be confused why the clearing is not
> done.
>
> @@ -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) >>
>

That sounds like a good idea!