On Tue, Aug 3, 2021 at 9:45 PM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Aug 3, 2021 at 12:18 PM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote: > > With this patch queue, fstest generic/208 (aio-dio-invalidate-failure.c) > > endlessly spins in gfs2_file_direct_write. It looks as if there's a bug > > in get_user_pages_fast when called with FOLL_FAST_ONLY: > > > > (1) The test case performs an aio write into a 32 MB buffer. > > > > (2) The buffer is initially not in memory, so when iomap_dio_rw() -> > > ... -> bio_iov_iter_get_pages() is called with the iter->noio flag > > set, we get to get_user_pages_fast() with FOLL_FAST_ONLY set. > > get_user_pages_fast() returns 0, which causes > > bio_iov_iter_get_pages to return -EFAULT. > > > > (3) Then gfs2_file_direct_write faults in the entire buffer with > > fault_in_iov_iter_readable(), which succeeds. > > > > (4) With the buffer in memory, we retry the iomap_dio_rw() -> > > ... -> bio_iov_iter_get_pages() -> ... -> get_user_pages_fast(). > > This should succeed now, but get_user_pages_fast() still returns 0. > > Hmm. Have you tried to figure out why that "still returns 0" happens? The call stack is: gup_pte_range gup_pmd_range gup_pud_range gup_p4d_range gup_pgd_range lockless_pages_from_mm internal_get_user_pages_fast get_user_pages_fast iov_iter_get_pages __bio_iov_iter_get_pages bio_iov_iter_get_pages iomap_dio_bio_actor iomap_dio_actor iomap_apply iomap_dio_rw gfs2_file_direct_write In gup_pte_range, pte_special(pte) is true and so we return 0. > One option - for debugging only - would be to introduce a new flag to > get_user_pages_fast() that says "print out reason if failed" and make > the retry (but not the original one) have that flag set. > > There are a couple of things of note when it comes to "get_user_pages_fast()": > > (a) some architectures don't even enable it > > (b) it can be very picky about the page table contents, and wants the > accessed bit to already be set (or the dirty bit, in the case of a > write). > > but (a) shouldn't be an issue on any common platform and (b) shouldn't > be an issue with fault_in_iov_iter_readable() that actually does a > __get_user() so it will access through the page tables. > > (It might be more of an issue with fault_in_iov_iter_writable() due to > walking the page tables by hand - if we don't do the proper > access/dirty setting, I could see get_user_pages_fast() failing). > > Anyway, for reason (a) I do think that eventually we should probably > introduce FOLL_NOFAULT, and allow the full "slow" page table walk - > just not calling down to handle_mm_fault() if it fails. > > But (a) should be a non-issue in your test environment, and so it > would be interesting to hear what it is that fails. Because scanning > through the patches, they all _look_ fine to me (apart from the one > comment about return values, which is more about being consistent with > copy_to/from_user() and making the code simpler - not about > correctness) > > Linus > Thanks, Andreas
