On 2020-05-18 21:55, John Hubbard wrote: > This code was using get_user_pages*(), in a "Case 2" scenario > (DMA/RDMA), using the categorization from [1]. That means that it's > time to convert the get_user_pages*() + put_page() calls to > pin_user_pages*() + unpin_user_pages() calls. > > There is some helpful background in [2]: basically, this is a small > part of fixing a long-standing disconnect between pinning pages, and > file systems' use of those pages. > > Note that this effectively changes the code's behavior as well: it now > ultimately calls set_page_dirty_lock(), instead of SetPageDirty().This > is probably more accurate. > > As Christoph Hellwig put it, "set_page_dirty() is only safe if we are > dealing with a file backed page where we have reference on the inode it > hangs off." [3] > > Also, this deletes one of the two FIXME comments (about refcounting), > because there is nothing wrong with the refcounting at this point. > > [1] Documentation/core-api/pin_user_pages.rst > > [2] "Explicit pinning of user-space pages": > https://lwn.net/Articles/807108/ > > [3] https://lore.kernel.org/r/20190723153640.GB720@xxxxxx Kai, why is the st driver calling get_user_pages_fast() directly instead of calling blk_rq_map_user()? blk_rq_map_user() is already used in st_scsi_execute(). I think that the blk_rq_map_user() implementation is also based on get_user_pages_fast(). See also iov_iter_get_pages_alloc() in lib/iov_iter.c. John, why are the get_user_pages_fast() calls in the st driver modified but not the blk_rq_map_user() call? Are you sure that the modified code is a "case 2" scenario and not a "case 1" scenario? Thanks, Bart.
