convert read_kcore(), vread() to use iterators

While reviewing Baoquan's recent changes to permit vread() access to vm_map_ram regions of vmalloc allocations, Willy pointed out [1] that it would be nice to refactor vread() as a whole, since its only user is read_kcore() and the existing form of vread() necessitates the use of a bounce buffer.

This patch series does exactly that, as well as adjusting how we read the kernel text section to avoid the use of a bounce buffer in this case as well.

This has been tested against the test case which motivated Baoquan's changes in the first place [2] which continues to function correctly, as do the vmalloc self tests.

[1] https://lore.kernel.org/all/Y8WfDSRkc%[email protected]/
[2] https://lore.kernel.org/all/[email protected]/T/#u

v8:
- Make zero_iter() static.

v7:
- Keep trying to fault in memory until the vmalloc read operation completes.
https://lore.kernel.org/all/[email protected]/

v6:
- Correct copy_page_to_iter_nofault() to handle -EFAULT case correctly.
https://lore.kernel.org/all/[email protected]/

v5:
- Do not rename fpos to ppos in read_kcore_iter() to avoid churn.
- Fix incorrect commit messages after prior revisions altered the approach.
- Replace copy_page_to_iter_atomic() with copy_page_to_iter_nofault() and adjust it to be able to handle compound pages. This uses copy_to_user_nofault() which ensures page faults are disabled during copy which kmap_local_page() was not doing.
- Only try to fault in pages if we are unable to copy in the first place and try only once to avoid any risk of spinning.
- Do not zero memory in aligned_vread_iter() if we couldn't copy it.
- Fix mistake in zeroing missing or unpopulated blocks in vmap_ram_vread_iter().
https://lore.kernel.org/linux-mm/[email protected]/

v4:
- Fixup mistake in email client which orphaned patch emails from the cover letter.
https://lore.kernel.org/all/[email protected]

v3:
- Revert introduction of mutex/rwsem in vmalloc
- Introduce copy_page_to_iter_atomic() iovec function
- Update vread_iter() and descendent functions to use only this
- Fault in user pages before calling vread_iter()
- Use const char* in vread_iter() and descendent functions
- Updated commit messages based on feedback
- Extend vread functions to always check how many bytes we could copy. If at any stage we are unable to copy/zero, abort and return the number of bytes we did copy.
https://lore.kernel.org/all/[email protected]/

v2:
- Fix ordering of vread_iter() parameters
- Fix nommu vread() -> vread_iter()
https://lore.kernel.org/all/[email protected]/

v1:
https://lore.kernel.org/all/[email protected]/

Lorenzo Stoakes (4):
fs/proc/kcore: avoid bounce buffer for ktext data fs/proc/kcore: convert read_kcore() to read_kcore_iter()
iov_iter: add copy_page_to_iter_nofault()
mm: vmalloc: convert vread() to vread_iter()

 fs/proc/kcore.c         |  85 +++++++--------
 include/linux/uio.h     |   2 +
 include/linux/vmalloc.h |   3 +-
 lib/iov_iter.c          |  48 +++++++++
 mm/nommu.c              |  10 +-
 mm/vmalloc.c            | 234 +++++++++++++++++++++++++---------------
 6 files changed, 243 insertions(+), 139 deletions(-)

--
2.39.2