Re: [PATCH v1 2/8] arm64, mm: transitional tables

On Thu, Aug 15, 2019 at 2:11 PM James Morse [off-list ref] wrote:

Hi Pavel,

On 01/08/2019 16:24, Pavel Tatashin wrote:

quoted

There are cases where normal kernel pages tables, i.e. idmap_pg_dir
and swapper_pg_dir are not sufficient because they may be overwritten.

This happens when we transition from one world to another: for example
during kexec kernel relocation transition, and also during hibernate
kernel restore transition.

In these cases, if MMU is needed, the page table memory must be allocated
from a safe place. Transitional tables is intended to allow just that.

quoted

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index db92950bb1a0..dcb4f13c7888 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h

@@ -110,6 +110,7 @@
 #define PUD_TABLE_BIT                (_AT(pudval_t, 1) << 1)
 #define PUD_TYPE_MASK                (_AT(pudval_t, 3) << 0)
 #define PUD_TYPE_SECT                (_AT(pudval_t, 1) << 0)
+#define PUD_SECT_RDONLY              (_AT(pudval_t, 1) << 7)         /* AP[2] */

This shouldn't be needed. As far as I'm aware, we only get read-only pages in the linear
map from debug-pagealloc, and the module aliases. Both of which require the linear map to
be made of page-size mappings.

Where are you seeing these?

This was done simply for generalization.

In old copy_pud:

445   set_pud(dst_pudp,
446   __pud(pud_val(pud) & ~PMD_SECT_RDONLY));

In new trans_table_copy_pud():
119                 if (!pud_table(src_pud)) {
120                         if (info->trans_flags & TRANS_MKWRITE)
121                                 pud_val(src_pud) &= ~PUD_SECT_RDONLY;

If you want, I can replace it with PMD_SECT_RDONLY

quoted

+ * trans_flags
+ *   - bitmap with flags that control how page table is filled.
+ *     TRANS_MKWRITE: during page table copy make PTE, PME, and PUD page
+ *                    writeable by removing RDONLY flag from PTE.
+ *     TRANS_MKVALID: during page table copy, if PTE present, but not valid,
+ *                    make it valid.
+ *     TRANS_CHECKPFN: During page table copy, for every PTE entry check that
+ *                     PFN that this PTE points to is valid. Otherwise return
+ *                     -ENXIO

Adding top-level global knobs to manipulate the copied linear map is going to lead to
bugs. The existing code will only change the PTE in specific circumstances, that it tests
for, that only happen at the PTE level.

I am simply doing the same what the old code is doing:

hibernate sets this flag if: debug_pagealloc_enabled() and trans_table
in this case check if pfn_valid() or not. This is part of
generalization, without which it is not possible to re-use the code
between kexec and hibernate.

quoted

+ *     TRANS_FORCEMAP: During page map, if translation exists, force
+ *                     overwrite it. Otherwise -ENXIO may be returned by
+ *                     trans_table_map_* functions if conflict is detected.

I will remove it.

Thank you,
Pasha

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