Messages in this thread | | | Date | Thu, 2 Jun 2022 17:46:48 +0100 | From | Matthew Wilcox <> | Subject | Re: [PATCH 0/3] btrfs: Replace kmap() with kmap_local_page() |
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On Thu, Jun 02, 2022 at 09:22:15AM -0700, Ira Weiny wrote: > On Wed, Jun 01, 2022 at 03:25:45PM +0200, David Sterba wrote: > > On Tue, May 31, 2022 at 04:53:32PM +0200, Fabio M. De Francesco wrote: > > > This is the first series of patches aimed towards the conversion of Btrfs > > > filesystem from the use of kmap() to kmap_local_page(). > > > > We've already had patches converting kmaps and you're changing the last > > ones, so this is could be the last series, with two exceptions. > > > > 1) You've changed lzo.c and zlib. but the same kmap/kunmap pattern is > > used in zstd.c. > > I checked out zstd.c and one of the issues there is the way that the input > workspace is mapped page by page while iterating those pages. > > This got me thinking about what Willy said at LSFmm concerning something like > kmap_local_range(). Mapping more than 1 page at a time could save some > unmap/remap of output pages required for kmap_local_page() ordering.
Umm ... Not entirely sure what I said, but it'd be really hard to kmap multiple pages with the current PAE implementation. I've steered away from doing that for now, and kmap_local_folio() just guarantees the page that the offset lands in is mapped.
I don't think the right answer is having a kmap_folio() that will map the entire folio. I'd be more tempted to add vmap_folio() for that. My understanding is that PAE systems have more address space available for vmap than they do for kmap.
> Unfortunately, I think the length of the input is probably to long in many > cases. And some remapping may still be required. > > Cc: Willy > > As an aside, Willy, I'm thinking that a kmap_local_range() should return a > folio in some way. Would you agree?
I imagine it taking a folio to describe the range that's being accessed. But maybe it should be a phys_addr_t? I tend to prefer phys_addr_t over pfn + offset as it's more compact on 64-bit systems and the same on 32-bit systems.
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