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

On Wed 21-12-22 19:04:02, Matthew Wilcox wrote:
> On Tue, Dec 20, 2022 at 03:59:39PM -0800, Ira Weiny wrote:
> > On Tue, Dec 20, 2022 at 06:34:57PM +0000, Matthew Wilcox wrote:
> > > 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'm confused.  A memcpy_to_folio() could loop to map the pages as needed
> > depending on the amount of data to copy.  Or just map/unmap in a loop.
> > 
> > This seems like an argument to have a memcpy_to_folio() to hide such nastiness
> > on HIGHMEM from the user.
> 
> I see that you are confused.  What I'm not quite sure of is how I confused
> you, so I'm just going to try again in different words.
> 
> Given the desire to support 2MB folios on x86/ARM PAE systems, we can't
> have a kmap_local_entire_folio() because that would take up too much
> address space.

Is that really a problem? I mean sure 2MB is noticeable in 32-bit address
space but these mappings are very shortlived due to their nature (and the
API kind of enforces that) so there'd hardly be more than a handful of them
existing in parallel on a system. Or is my expectation wrong?

But I agree the solution with memcpy_to/from_folio() works as well.

> > [*] I only play a file system developer on TV.  ;-)
> 
> That's OK, I'm only pretending to be an MM developer.  Keep quiet, and
> I think we can get away with this.

"All the world's a stage, and all the men and women merely players." :)

								Honza

-- 
Jan Kara <[email protected]>
SUSE Labs, CR