Re: [PATCH] mm: introduce kvvirt_to_page() helper

[Michal Hocko <mhocko@kernel.org>]

On Mon 20-08-18 10:07:44, Matthew Wilcox wrote:
> On Mon, Aug 20, 2018 at 06:24:06PM +0200, Michal Hocko wrote:
> > On Mon 20-08-18 07:49:23, Matthew Wilcox wrote:
> > > On Mon, Aug 20, 2018 at 04:41:16PM +0200, Michal Hocko wrote:
> > > > On Sat 18-08-18 20:49:01, Li RongQing wrote:
> > > > > The new helper returns address mapping page, which has several users
> > > > > in individual subsystem, like mem_to_page in xfs_buf.c and pgv_to_page
> > > > > in af_packet.c, unify them
> > > > 
> > > > kvvirt_to_page is a weird name. I guess you wanted it to fit into
> > > > kv*alloc, kvfree naming, right? If yes then I guess kvmem_to_page
> > > > would be slightly better.
> > > > 
> > > > Other than that the patch makes sense to me. It would be great to add
> > > > some documentation and be explicit that the call is only safe on
> > > > directly mapped kernel address and vmalloc areas.
> > > 
> > > ... and not safe if the length crosses a page boundary.  I don't want to
> > > see new code emerge that does kvmalloc(PAGE_SIZE * 2, ...); kvmem_to_page()
> > > and have it randomly crash when kvmalloc happens to fall back to vmalloc()
> > > under heavy memory pressure.
> > > 
> > > Also, people are going to start using this for stack addresses.  Perhaps
> > > we should have a debug option to guard against them doing that.
> > 
> > I do agree that such an interface is quite dangerous. That's why I was
> > stressing out the proper documentation. I would be much happier if we
> > could do without it altogether. Maybe the existing users can be rewoked
> > to not rely on the addr2page functionality. If that is not the case then
> > we should probably offer a helper. With some WARN_ONs to catch misuse
> > would be really nice. I am not really sure how many abuses can we catch
> > actually though.
> 
> I certainly understand the enthusiasm for sharing this code rather than
> having dozens of places outside the VM implement their own version of it.
> But I think most of these users are using code that's working at the wrong
> level.  Most of them seem to have an address range which may-or-may-not-be
> virtually mapped and they want to get an array-of-pages for that.
> 
> Perhaps we should offer -that- API instead.  vmalloc/vmap already has
> an array-of-pages, and the various users could be given a pointer to
> that array.  If the memory isn't vmapped, maybe the caller could pass
> an array pointer like XFS does, or we could require them to pass GFP flags
> to allocate a new array.

Sure, I wouldn't be opposed if there was a model which doesn't force
them to do hacks like this.

-- 
Michal Hocko
SUSE Labs