Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present

[Lance Yang <lance.yang@linux.dev>]

On 2026/10/5 21:46, Lance Yang wrote:
> 
> On Mon, Oct 05, 2026 at 12:37:20PM +0200, David Hildenbrand (Arm) wrote:
>> On 10/4/26 05:03, Lance Yang wrote:
>>> RISC-V uses _PAGE_EXEC for swap soft-dirty tracking when
>>> CONFIG_MEM_SOFT_DIRTY is enabled and Svrsw60t59b is available. That's
>>> a problem for PMD migration entries, since pmd_present() also checks
>>> _PAGE_LEAF (R/W/X) to recognize THPs with _PAGE_PRESENT temporarily
>>> cleared during splitting.
>>
>> I'm curious: why do we have to set leaf indications for non-present things? The
>> HW sure will ignore it, right?
> 
> Yeah, that surprised me too :) Still wrapping my head around the details
> ...
> 
>> Is this a sw problem? Who needs that?
> 
> IIUC, it's for software during a PMD split.
> 
> __split_huge_pmd_locked() invalidates the huge PMD and flushes the TLB
> before installing the PTE table. Software still needs pmd_present() and
> pmd_trans_huge() to recognize the THP in between.

Also, x86 keeps _PAGE_PSE to identify the huge PMD. RISC-V doesn't
have a separate leaf bit, so it keeps the R/W/X bits to identify
the PMD as a leaf entry :)

> 
> RISC-V clears V but keeps the R/W/X bits for that, so the entry is invalid
> to hardware but still identifiable as a THP by software.
> 
> Hopefully I didn't miss something.
> 
>>>
>>> When a soft-dirty THP is migrated, set_pmd_migration_entry() preserves
>>> soft-dirty with pmd_swp_mksoft_dirty(), setting the X bit in the
>>> migration PMD. Even with _PAGE_PRESENT clear, we end up treating a
>>> migration PMD as a present THP! The fault handler skips
>>> pmd_migration_entry_wait(), and a write fault can end up in
>>> do_huge_pmd_wp_page(), where pmd_page() decodes the migration entry
>>> as a mapped PFN.
>>
>> That sounds bad.
> 
> YES, looks a bit off ...
> 
>>>
>>> Move the swap soft-dirty bit to bit 12 and start the swap offset at
>>> bit 13 when CONFIG_MEM_SOFT_DIRTY is enabled. This keeps R/W/X clear
>>> in migration PMDs and lets us keep the existing pmd_present() check
>>> for invalidated THPs. Leave the offset at bit 12 when soft-dirty
>>> tracking is disabled.
>>
>> That reduces the effective swap size (and PFN we can store). Could that be a
>> problem?
> 
> We don't need all 52 bits of the swap offset.
> 
> RV64 PFNs only need 44 bits for migration entries, and actual swap is
> already limited to about 16 TiB per area with 4 KiB pages by
> last_page (__u32) and swap_info_struct.max (unsigned int).
> 
> So there's room to reserve a bit without reducing the supported swap
> size or PFN range.
> 
> CONFIG_MEM_SOFT_DIRTY is only available on RV64, so RV32 keeps its 20-bit
> offset.
> 
> [...]
> 
> Cheers, Lance