On Mon, Mar 31, 2025 at 06:45:10PM +1030, Qu Wenruo wrote: > 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. page_offset() is on my hitlist. It actually is correct now (commit 12851bd921d4) but it's on its way out. Don't use bv_page. > 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. I have no idea what this means. > 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? I don't understand your requirements. but doing something different that fills in a folio_iter along the lines of bio_for_each_folio_all() would make sense.
