linux-kernel - Re: [PATCH 2/2] vfree: Update documentation

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Date:   Tue, 22 Sep 2020 17:09:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH 2/2] vfree: Update documentation

On Tue, Sep 22, 2020 at 04:06:03PM +0100, Matthew Wilcox wrote:
> I don't think it makes sense to list all vmalloc-style allocators here.
> It won't be updated by people who add new variations.  How about this?
> 
>  * Free the virtually continuous memory area starting at @addr, as
>  * obtained from one of the vmalloc() family of APIs.  This will
>  * usually also free the physical memory underlying the virtual
>  * allocation, but that memory is reference counted, so it will not
>  * be freed until the last user goes away.
>  *
>  * If @addr is NULL, no operation is performed.
> 
> I'm trying to strike a balance between being accurate and not requiring
> device driver authors to learn all about struct page.  I may be too
> close to the implementation to write good documentation for it.

I think the above is sensible, but not enough.  vmap really needs to
be treated special, as by default area->pages for vmap is NULL.  So
for vfree to be useful on a vmap mapping, the callers needs to
manually set it up by poking into the internals.  Actually, I think
we really want another API rather than vmap for that.  Let me respin
my series to include that.