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
