On Wed, Jul 12, 2023 at 09:31:39AM +0530, Anshuman Khandual wrote: > On 7/10/23 16:55, Mark Rutland wrote: > > On Fri, Jul 07, 2023 at 11:03:27AM +0530, Anshuman Khandual wrote: > >> These pte_dirty() changes make things explicitly clear, while improving the > >> code readability. This optimizes HW dirty state transfer into SW dirty bit. > >> This also adds a new arm64 documentation explaining overall pte dirty state > >> management in detail. This series applies on the latest mainline kernel. > > > > TBH, I think this is all swings and roundabouts, and I'm not sure this is > > worthwhile. I appreciate that as-is some people find this confusing, but I I'm pretty much on the same lines, though maybe I looked too much at this code that I don't like any further changes to it ;). > Current situation for pte_dirty() management is confusing when there are two > distinct mechanisms to track PTE dirty states, but both are forced to work > together because > > - HW DBM cannot track non-writable dirty state (PTE_DBM == PTE_WRITE) > - Runtime check for HW DBM is avoided Depending on how you look at it, we can say that any writeable PTE (as in page table permission, PTE_RDONLY cleared) is dirty and we only have a software mechanism for tracking the dirty state. The DBM feature is not actually giving us a dirty bit but an automated way to make a PTE writeable on access (for some historical reasons like the SMMU not having such mechanism in place). Maybe we can clean the code a bit based on the above perspective. E.g. instead of pte_hw_dirty() just have a !pte_hw_rdonly() macro. It may help with the confusion of having two mechanisms. OTOH, with PIE, we can have a true dirty bit but at that point we can eliminate the pte_sw_dirty() use entirely and allow soft-dirty using the current PTE_DIRTY (with some static labels based on the feature). > > don't think the end result of this series is actually better, and it adds more > > code/documentation to maintain. > > Agreed, it does add more code and documentation but still trying to understand > why it is not worthwhile. Regardless, following patch does optimize a situation > where we dont need to call pte_mkdirty() knowing it will be cleared afterwards. > > [RFC 2/4] arm64/mm: Call pte_sw_mkdirty() while preserving the HW dirty state I wonder whether the compiler eliminates much of this duplication since there are some checks for pte_write() before. We may be able to remove some checks. For example, does pte_hw_dirty() actually need to check pte_write()? A !PTE_RDONLY entry is dirty automatically since we can't trap any write access to it (prior to PIE; I need to check Joey's patches on how it treats writeable+clean PTEs; still on holiday). As for the fourth patch, I'd rather add documentation in the header file, it's more likely to be looked at and updated. -- Catalin
