Re: [PATCH 5/8] reiserfs: Convert do_journal_end() to use kmap_local_folio()

On Tue, Dec 20, 2022 at 08:58:52AM -0800, Ira Weiny wrote:
> On Tue, Dec 20, 2022 at 12:18:01PM +0100, Jan Kara wrote:
> > On Tue 20-12-22 09:35:43, Matthew Wilcox wrote:
> > > But that doesn't solve the "What about fs block size > PAGE_SIZE"
> > > problem that we also want to solve.  Here's a concrete example:
> > > 
> > >  static __u32 jbd2_checksum_data(__u32 crc32_sum, struct buffer_head *bh)
> > >  {
> > > -       struct page *page = bh->b_page;
> > > +       struct folio *folio = bh->b_folio;
> > >         char *addr;
> > >         __u32 checksum;
> > >  
> > > -       addr = kmap_atomic(page);
> > > -       checksum = crc32_be(crc32_sum,
> > > -               (void *)(addr + offset_in_page(bh->b_data)), bh->b_size);
> > > -       kunmap_atomic(addr);
> > > +       BUG_ON(IS_ENABLED(CONFIG_HIGHMEM) && bh->b_size > PAGE_SIZE);
> > > +
> > > +       addr = kmap_local_folio(folio, offset_in_folio(folio, bh->b_data));
> > > +       checksum = crc32_be(crc32_sum, addr, bh->b_size);
> > > +       kunmap_local(addr);
> > >  
> > >         return checksum;
> > >  }
> > > 
> > > I don't want to add a lot of complexity to handle the case of b_size >
> > > PAGE_SIZE on a HIGHMEM machine since that's not going to benefit terribly
> > > many people.  I'd rather have the assertion that we don't support it.
> > > But if there's a good higher-level abstraction I'm missing here ...
> > 
> > Just out of curiosity: So far I was thinking folio is physically contiguous
> > chunk of memory. And if it is, then it does not seem as a huge overkill if
> > kmap_local_folio() just maps the whole folio?
> 
> Willy proposed that previously but we could not come to a consensus on how to
> do it.
> 
> https://lore.kernel.org/all/Yv2VouJb2pNbP59m@iweiny-desk3/
> 
> FWIW I still think increasing the entries to cover any foreseeable need would
> be sufficient because HIGHMEM does not need to be optimized.  Couldn't we hide
> the entry count into some config option which is only set if a FS needs a
> larger block size on a HIGHMEM system?

"any foreseeable need"?  I mean ... I'd like to support 2MB folios,
even on HIGHMEM machines, and that's 512 entries.  If we're doing
memcpy_to_folio(), we know that's only one mapping, but still, 512
entries is _a lot_ of address space to be reserving on a 32-bit machine.
I don't know exactly what the address space layout is on x86-PAE or
ARM-PAE these days, but as I recall, the low 3GB is user and the high
1GB is divided between LOWMEM and VMAP space; something like 800MB of
LOWMEM and 200MB of vmap/kmap/PCI iomem/...

Where I think we can absolutely get away with this reasoning is having
a kmap_local_buffer().  It's perfectly reasonable to restrict fs block
size to 64kB (after all, we've been limiting it to 4kB on x86 for thirty
years), and having a __kmap_local_pfns(pfn, n, prot) doesn't seem like
a terribly bad idea to me.

So ... is this our path forward:

 - Introduce a complex memcpy_to/from_folio() in highmem.c that mirrors
   zero_user_segments()
 - Have a simple memcpy_to/from_folio() in highmem.h that mirrors
   zero_user_segments()
 - Convert __kmap_local_pfn_prot() to __kmap_local_pfns()
 - Add kmap_local_buffer() that can handle buffer_heads up to, say, 16x
   PAGE_SIZE