So we've had this regression in 9p for.. almost a year, which is way too long, but there was no "easy" reproducer until yesterday (thank you again!!) It turned out to be a bug with iov_iter on folios, iov_iter_get_pages_alloc2() would advance the iov_iter correctly up to the end edge of a folio and the later copy_to_iter() fails on the iterate_folioq() bug. Happy to consider alternative ways of fixing this, now there's a reproducer it's all much clearer; for the bug to be visible we basically need to make and IO with non-contiguous folios in the iov_iter which is not obvious to test with synthetic VMs, with size that triggers a zero-copy read followed by a non-zero-copy read. Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx> --- Changes in v2: - Fixed 'remain' being used uninitialized in iterate_folioq when going through the goto - s/forwarded/advanced in commit message - Link to v1: https://lore.kernel.org/r/20250811-iot_iter_folio-v1-0-d9c223adf93c@xxxxxxxxxxxxx --- Dominique Martinet (2): iov_iter: iterate_folioq: fix handling of offset >= folio size iov_iter: iov_folioq_get_pages: don't leave empty slot behind include/linux/iov_iter.h | 5 ++++- lib/iov_iter.c | 6 +++--- 2 files changed, 7 insertions(+), 4 deletions(-) --- base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585 change-id: 20250811-iot_iter_folio-1b7849f88fed Best regards, -- Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
