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? Or are you concerned about
the overhead of finding big enough hole in the vmap area?
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
