On Tue, Nov 05, 2019 at 10:29:03AM +0000, Will Deacon wrote:

On Mon, Nov 04, 2019 at 05:16:42PM -0800, John Stultz wrote:

quoted

On Tue, Oct 29, 2019 at 8:31 AM Catalin Marinas [off-list ref] wrote:

quoted

Shared and writable mappings (__S.1.) should be clean (!dirty) initially
and made dirty on a subsequent write either through the hardware DBM
(dirty bit management) mechanism or through a write page fault. A clean
pte for the arm64 kernel is one that has PTE_RDONLY set and PTE_DIRTY
clear.

The PAGE_SHARED{,_EXEC} attributes have PTE_WRITE set (PTE_DBM) and
PTE_DIRTY clear. Prior to commit 73e86cb03cf2 ("arm64: Move PTE_RDONLY
bit handling out of set_pte_at()"), it was the responsibility of
set_pte_at() to set the PTE_RDONLY bit and mark the pte clean if the
software PTE_DIRTY bit was not set. However, the above commit removed
the pte_sw_dirty() check and the subsequent setting of PTE_RDONLY in
set_pte_at() while leaving the PAGE_SHARED{,_EXEC} definitions
unchanged. The result is that shared+writable mappings are now dirty by
default

Fix the above by explicitly setting PTE_RDONLY in PAGE_SHARED{,_EXEC}.
In addition, remove the superfluous PTE_DIRTY bit from the kernel PROT_*
attributes.

Fixes: 73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()")
Cc: <redacted> # 4.14.x-
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

[...]

As an experiment, can you try reverting just the part of the patch that
removes PTE_DIRTY from the PROT_* definitions? (see below)

Another thing worth trying is reverting commit 747a70e60b72 ("arm64: Fix
copy-on-write referencing in HugeTLB") when this patch is applied. That
commit is not just about hugetlb but changes pte_same() to ignore
PTE_RDONLY on the assumption that this is set by set_pte_at(). We
subsequently changed set_pte_at() to drop PTE_RDONLY.

-- 
Catalin

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