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@codewreck.org --- Changes in v3: - convert 'goto next' to a big "if there is valid data in current folio" Future optimizations can remove it again after making sure this (iov iter advanced to end of folio) can never happen. - Link to v2: https://lore.kernel.org/r/20250812-iot_iter_folio-v2-0-f99423309478@codewrec...
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@codewrec...
--- 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 | 20 +++++++++++--------- lib/iov_iter.c | 6 +++--- 2 files changed, 14 insertions(+), 12 deletions(-) --- base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585 change-id: 20250811-iot_iter_folio-1b7849f88fed
Best regards,