Jens Axboe <axboe@xxxxxxxxx> wrote: > > A field, bi_cleanup_mode, is added to the bio struct that gets set by > > iov_iter_extract_pages() with FOLL_* flags indicating what cleanup is > > necessary. FOLL_GET -> put_page(), FOLL_PIN -> unpin_user_page(). Other > > flags could also be used in future. > > > > Newly allocated bio structs have bi_cleanup_mode set to FOLL_GET to > > indicate that attached pages are ref'd by default. Cloning sets it to 0. > > __bio_iov_iter_get_pages() overrides it to what iov_iter_extract_pages() > > indicates. > > What's the motivation for this change? DIO reads in most filesystems and, I think, the block layer are currently broken with respect to concurrent fork in the same process because they take refs (FOLL_GET) on the pages involved which causes the CoW mechanism to malfunction, leading (I think) the parent process to not see the result of the DIO. IIRC, the pages undergoing DIO get forcibly copied by fork - and the copies given to the parent. Instead, DIO reads should be pinning the pages (FOLL_PIN). Maybe Willy can weigh in on this? Further, getting refs on pages in, say, a KVEC iterator is the wrong thing to do as the kvec may point to things that shouldn't be ref'd (vmap'd or vmalloc'd regions, for example). Instead, the in-kernel caller should do what it needs to do to keep hold of the memory and the DIO should not take a ref at all. > It's growing struct bio, which we can have a lot of in the system. I read > the cover letter too and I can tell what the change does, but there's no > justification really for the change. The FOLL_* flags I'm getting back from iov_iter_extract_pages() can be mapped to BIO_* flags in the bio. For the moment, AFAIK, I think only FOLL_GET and FOLL_PIN are necessary. There are three cleanup types: put, unpin and do nothing. David
