On Tue, Nov 5, 2019 at 8:54 AM Catalin Marinas [off-list ref] wrote:

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

quoted

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>

[...]

quoted

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.

Just to confirm, reverting 747a70e60b72 instead of aa57157be69f also
seems to avoid the issue I'm seeing.

I've not tried Will patch but I'll do that next. Though its not clear
if you wanted me to revert 747a70e60b72 on top of Will's test patch or
not?

thanks
-john

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