[PATCHv3 0/2] Fix SIGBUS semantics with large folios

[Kiryl Shutsemau <kirill@shutemov.name>]

From: Kiryl Shutsemau <kas@kernel.org>

Accessing memory within a VMA, but beyond i_size rounded up to the next
page size, is supposed to generate SIGBUS.

Darrick reported[1] an xfstests regression in v6.18-rc1. generic/749
failed due to missing SIGBUS. This was caused by my recent changes that
try to fault in the whole folio where possible:

        19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()")
        357b92761d94 ("mm/filemap: map entire large folio faultaround")

These changes did not consider i_size when setting up PTEs, leading to
xfstest breakage.

However, the problem has been present in the kernel for a long time -
since huge tmpfs was introduced in 2016. The kernel happily maps
PMD-sized folios as PMD without checking i_size. And huge=always tmpfs
allocates PMD-size folios on any writes.

I considered this corner case when I implemented a large tmpfs, and my
conclusion was that no one in their right mind should rely on receiving
a SIGBUS signal when accessing beyond i_size. I cannot imagine how it
could be useful for the workload.

But apparently filesystem folks care a lot about preserving strict
SIGBUS semantics.

Generic/749 was introduced last year with reference to POSIX, but no
real workloads were mentioned. It also acknowledged the tmpfs deviation
from the test case.

POSIX indeed says[3]:

        References within the address range starting at pa and
        continuing for len bytes to whole pages following the end of an
        object shall result in delivery of a SIGBUS signal.

The patchset fixes the regression introduced by recent changes as well
as more subtle SIGBUS breakage due to split failure on truncation.

v3:
 - Make an exception for tmpfs/shmem, code restructured;
 - Rebased to mm-everything (v2 of the patchset reverted);
v2:
 - Fix try_to_unmap() flags;
 - Add warning if try_to_unmap() fails to unmap the folio;
 - Adjust comments and commit messages;
 - Whitespace fixes;
v1:
 - Drop RFC;
 - Add Signed-off-bys;

Kiryl Shutsemau (2):
  mm/memory: Do not populate page table entries beyond i_size
  mm/truncate: Unmap large folio on split failure

 mm/filemap.c  | 28 ++++++++++++++++++++--------
 mm/memory.c   | 20 +++++++++++++++++++-
 mm/truncate.c | 35 +++++++++++++++++++++++++++++------
 3 files changed, 68 insertions(+), 15 deletions(-)

-- 
2.50.1