On Wed, May 18, 2022 at 01:56:31PM +0300, Jasper Surmont wrote: > I'm writing a device-mapper which has to edit incoming writes, for > example: Incoming bio request contains a biovec with a page full of 0 > bytes, and I want to change it to all 0xFF bytes. > > I tried using bvec_kmap_local() to get the page address, to then read > the data and if needed adjust it using memcpy or similar. Initial > tests seemed to work, but if I execute things like mkfs on the created > dm, I get a lot of segmentation faults. I narrowed down the problem > that this only happens if I actually write something to the mapped > page. However, I see no reason why it should cause this fault as I > (after checking probably 100 times) don't access invalid memory. I'm > wondering whether my method of editing the write is actually correct, > or if I am doing something else very stupid. Data in bios can't be modified in-place, as submitters of writes (e.g., filesystems) expect that their data buffers aren't modified. To modify the data you'd need to allocate a new bio with new pages and submit that instead. See dm-crypt for an example of a device-mapper target which does this. What sort of device-mapper target are you trying to implement, anyway? If you're trying to do encryption, dm-crypt already solves that problem. - Eric
