Hello, Thank you for replying. On 11/14/22 8:46 PM, David Hildenbrand wrote: >> The soft-dirtiness is stored in the PTE. VMA is marked dirty to store the >> dirtiness for reused regions. Clearing the soft-dirty status of whole >> process is straight forward. When we want to clear/monitor the >> soft-dirtiness of a part of the virtual memory, there is a lot of internal >> noise. We don't want the non-dirty pages to become dirty because of how the >> soft-dirty feature has been working. Soft-dirty feature wasn't being used >> the way we want to use now. While monitoring a part of memory, it is not >> acceptable to get non-dirty pages as dirty. Non-dirty pages become dirty >> when the two VMAs are merged without considering if they both are dirty or >> not (34228d473efe). To monitor changes over the memory, sometimes VMAs are >> split to clear the soft-dirty bit in the VMA flags. But sometimes kernel >> decide to merge them backup. It is so waste of resources. > > Maybe you'd want a per-process option to not merge if the VM_SOFTDIRTY > property differs. But that might be just one alternative for handling this > case. > >> >> To keep things consistent, the default behavior of the IOCTL is to output >> even the extra non-dirty pages as dirty from the kernel noise. A optional >> PAGEMAP_NO_REUSED_REGIONS flag is added for those use cases which aren't >> tolerant of extra non-dirty pages. This flag can be considered as something >> which is by-passing the already present buggy implementation in the kernel. >> It is not buggy per say as the issue can be solved if we don't allow the >> two VMA which have different soft-dirty bits to get merged. But we are >> allowing that so that the total number of VMAs doesn't increase. This was >> acceptable at the time, but now with the use case of monitoring a part of >> memory for soft-dirty doesn't want this merging. So either we need to >> revert 34228d473efe and PAGEMAP_NO_REUSED_REGIONS flag will not be needed >> or we should allow PAGEMAP_NO_REUSED_REGIONS or similar mechanism to ignore >> the extra dirty pages which aren't dirty in reality. >> >> When PAGEMAP_NO_REUSED_REGIONS flag is used, only the PTEs are checked to >> find if the pages are dirty. So re-used regions cannot be detected. This >> has the only side-effect of not checking the VMAs. So this is limitation of >> using this flag which should be acceptable in the current state of code. >> This limitation is okay for the users as they can clear the soft-dirty bit >> of the VMA before starting to monitor a range of memory for soft-dirtiness. >> >> >>> Please separate that part out from the other changes; I am still not >>> convinced that we want this and what the semantical implications are. >>> >>> Let's take a look at an example: can_change_pte_writable() >>> >>> /* Do we need write faults for softdirty tracking? */ >>> if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte)) >>> return false; >>> >>> We care about PTE softdirty tracking, if it is enabled for the VMA. >>> Tracking is enabled if: vma_soft_dirty_enabled() >>> >>> /* >>> * Soft-dirty is kind of special: its tracking is enabled when >>> * the vma flags not set. >>> */ >>> return !(vma->vm_flags & VM_SOFTDIRTY); >>> >>> Consequently, if VM_SOFTDIRTY is set, we are not considering the soft_dirty >>> PTE bits accordingly. >> Sorry, I'm unable to completely grasp the meaning of the example. We have >> followed clear_refs_write() to write the soft-dirty bit clearing code in >> the current patch. Dirtiness of the VMA and the PTE may be set >> independently. Newer allocated memory has dirty bit set in the VMA. When >> something is written the memory, the soft dirty bit is set in the PTEs as >> well regardless if the soft dirty bit is set in the VMA or not. >> > > Let me try to find a simple explanation: > > After clearing a SOFTDIRTY PTE flag inside an area with VM_SOFTDIRTY set, > there are ways that PTE could get written to and it could become dirty, > without the PTE becoming softdirty. > > Essentially, inside a VMA with VM_SOFTDIRTY set, the PTE softdirty values > might be stale: there might be entries that are softdirty even though the > PTE is *not* marked softdirty. Can someone please share the example to reproduce this? In all of my testing, even if I ignore VM_SOFTDIRTY and only base my decision of soft-dirtiness on individual pages, it always passes. > > These are, AFAIU, the current semantics, and I am not sure if we want user > space to explicitly work around that. > >>> >>> >>> I'd suggest moving forward without this controversial >>> PAGEMAP_NO_REUSED_REGIONS functionality for now, and preparing it as a >>> clear add-on we can discuss separately.Like I've described above, I've >>> only added this flag to not get the >> non-dirty pages as dirty. Can there be some alternative to adding this >> flag? Please suggest. > > Please split it out into a separate patch for now. We can discuss further > what the semantics are and if there are better alternatives for that. In > the meantime, you could move forward without PAGEMAP_NO_REUSED_REGIONS > while we are discussing them further. > -- BR, Muhammad Usama Anjum
