Re: folio_map

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 17, 2022 at 01:29:35PM +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 16, 2022 at 07:08:22PM +0100, 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.
> > 
> > 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);
> > +}
> > +
> > +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)
> 
> So it aims at replacing kmap_local_page(), but for folios, right?
> kmap_local_page() interface can be used from any context, but vmap helpers
> might_sleep(). How do we rectify this?

I'm not proposing getting rid of kmap_local_folio().  That should still
exist and work for users who need to use it in atomic context.  Indeed,
I'm intending to put a note in the doc for folio_map_local() suggesting
that users may prefer to use kmap_local_folio().  Good idea to put a
might_sleep() in folio_map_local() though.