== SRU Justification ==
IBM is requesting these commits in bionic and cosmic. These commits
also rely on commit 7acf50e4efa6, which was SRU'd in bug 1792102.
Description of bug:
GPFS mmfsd daemon is mapping shared tracing buffer(allocated from kernel
driver using vmalloc) and then writing trace records from user space threads
in parallel. While the SIGBUS happened, the access virtual memory address
is in the mapped range, no overflow on access.
The root cause is that for PTEs created by a driver at mmap time (ie, that
aren't created dynamically at fault time), it's not legit for ptep_set_access_flags()
to make them invalid even temporarily. A concurrent access while they are
invalid will be unable to service the page fault and will cause as SIGBUS.
== Fixes ==
bd0dbb73e013 ("powerpc/mm/books3s: Add new pte bit to mark pte temporarily invalid.")
f08d08f3db55 ("powerpc/mm/radix: Only need the Nest MMU workaround for R -> RW transition")
== Regression Potential ==
Low. Limited to powerpc.
== Test Case ==
A test kernel was built with these patches and tested by IBM.
IBM states the test kernel resolved the bug.
-- Problem Description --
GPFS mmfsd daemon is mapping shared tracing buffer(allocated from kernel driver using vmalloc) and then writing trace records from user space threads in parallel. While the SIGBUS happened, the access virtual memory address is in the mapped range, no overflow on access.
Worked with Benjamin Herrenschmidt on GPFS tracing kernel driver code and he made a suggestion as workaround on the driver code to bypass the problem, and it works....
the workaround code change as below:
- rc = remap_pfn_range(vma, start, pfn, PAGE_SIZE, PAGE_SHARED);
+ rc = remap_pfn_range(vma, start, pfn, PAGE_SIZE, __pgprot(pgprot_val(PAGE_SHARED)|_PAGE_DIRTY);
As Benjamin mentioned, this is a Linux kernel bug and this is just a workaround. He will give the details about the kernel bug and why this workaround works....
The root cause is that for PTEs created by a driver at mmap time (ie, that aren't created dynamically at fault time), it's not legit for ptep_set_access_flags() to make them invalid even temporarily. A concurrent access while they are invalid will be unable to service the page fault and will cause as SIGBUS.
Thankfully such PTEs shouldn't normally be the subject of a RO->RW privilege escalation.
What happens is that the GPFS driver creates the PTEs using remap_pfn_range(...,PAGE_SHARED).
PAGE_SHARED has _PAGE_ACCESSED (R) but not _PAGE_DIRTY (C) set.
Thus on the first write, we try set C and while doing so, hit the above workaround, which causes the problem described earlier.
The proposed patch will ensure we only do the Nest MMU hack when changing _PAGE_RW and not for normal R/C updates.
The workaround tested by the GPFS team consists of adding _PAGE_DIRTY to the mapping created by remap_pfn_range() to avoid the RC update fault completely.
This is fixed by these:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bd0dbb73e01306a1060e56f81e5fe287be936477
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f08d08f3db55452d31ba4a37c702da6245876b96
Since DD1 support is still in (ie, 2bf1071a8d50928a4ae366bb3108833166c2b70c is not applied) the second doesn't apply cleanly. Did you want that attached?
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