Re: folio_map

[John Hubbard <jhubbard@nvidia.com>]

On 8/16/22 11:08, Matthew Wilcox wrote:
> Some of you will already know all this, but I'll go into a certain amount
> of detail for the peanut gallery.
> 
> One of the problems that people want to solve with multi-page folios
> is supporting filesystem block sizes > PAGE_SIZE.  Such filesystems
> already exist; you can happily create a 64kB block size filesystem on
> a PPC/ARM/... today, then fail to mount it on an x86 machine.
> 
> kmap_local_folio() only lets you map a single page from a folio.
> This works for the majority of cases (eg ->write_begin() works on a
> per-page basis *anyway*, so we can just map a single page from the folio).
> But this is somewhat hampering for ext2_get_page(), used for directory
> handling.  A directory record may cross a page boundary (because it
> wasn't a page boundary on the machine which created the filesystem),
> and juggling two pages being mapped at once is tricky with the stack
> model for kmap_local.
> 
> I don't particularly want to invest heavily in optimising for HIGHMEM.
> The number of machines which will use multi-page folios and HIGHMEM is
> not going to be large, one hopes, as 64-bit kernels are far more common.
> I'm happy for 32-bit to be slow, as long as it works.

Some of our kernel driver teams recently expressed precisely the same set
of requirements. And at first, I pointed them to folio_map_local(),
and then they schooled me by noting that, today, it only does a single
page. :)

> 
> For these reasons, I proposing the logical equivalent to this:
> 
> +void *folio_map_local(struct folio *folio)
> +{
> +       if (!IS_ENABLED(CONFIG_HIGHMEM))
> +               return folio_address(folio);
> +       if (!folio_test_large(folio))
> +               return kmap_local_page(&folio->page);
> +       return vmap_folio(folio);
> +}

...which led to a desire for code very much like the above: kmap(),
with a fallback to vmap(). Always better to have such things in the
kernel, rather than a zillion copies in drivers.

Adding Mark Hairgrove in case I've missed any fine points?

> +
> +void folio_unmap_local(const void *addr)
> +{
> +       if (!IS_ENABLED(CONFIG_HIGHMEM))
> +               return;
> +       if (is_vmalloc_addr(addr))
> +               vunmap(addr);
> +	else
> +       	kunmap_local(addr);
> +}
> 
> (where vmap_folio() is a new function that works a lot like vmap(),
> chunks of this get moved out-of-line, etc, etc., but this concept)
> 
> Does anyone have any better ideas?  If it'd be easy to map N pages
> locally, for example ... looks like we only support up to 16 pages
> mapped per CPU at any time, so mapping all of a 64kB folio would
> almost always fail, and even mapping a 32kB folio would be unlikely
> to succeed.
> 
> 

thanks,
-- 
John Hubbard
NVIDIA