Hi, The seemingly easy question has some very interesting extra requirements: 1. The bio contains contig file map folios The folios may be large. So page_offset() on bv_page (using single-page bvec) is no longer reliable, one has to call page_pgoff() instead. 2. The data may not cover the bio range So we need some range comparison and skip if the data range doesn't cover the bio range. 3. The bio may have been advanced E.g. previous de-compressed range has been copied, but the remaining part still needs to be fulfilled. And we need to use the bv_page's file offset to calculate the real beginning of the range to copy. The current btrfs code is doing single page bvec iteration, and handling point 2 and 3 well. (btrfs_decompress_buf2page() in fs/btrfs/compression.c) Point 1 was not causing problem until the incoming large data folio support, and can be easily fixed with page_pgoff() convertion. But since we're here, I'm also wondering can we do it better with a folio or multi-page bvec way? The current folio bio iteration helper can only start from the beginning of a bio (bio_for_each_folio_all() and bio_first_folio()), thus it's not a good fit for point 3. On the other hand, I'm having some internal code to convert a bio_vec into a folio and offset inside the folio already. Thus I'm wondering can we provide something like bio_for_each_folio()? Or is it too niche that only certain fs can benefit from? Thanks, Qu
