Re: [PATCH 2/2] vfree: Update documentation

From: Matthew Wilcox
Date: Tue Sep 22 2020 - 11:22:45 EST

On Tue, Sep 22, 2020 at 05:09:10PM +0200, Christoph Hellwig wrote:
> 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.

I've been thinking about somethng like:

void *vmap_mapping(struct address_space *mapping, pgoff_t start,
unsigned long len);

but it doesn't quite work for the shmem cases because they need to use
shmem_read_mapping_page_gfp() instead of ->readpage. I'd also want it
to work for DAX, but I don't have a user for that yet so it's hard to
justify adding it.

I have ideas for making shmem/tmpfs work more like other filesystems,
so we don't need shmem_read_mapping_page_gfp() but there are only so
many hours in the day.