On 11/21/22 8:55 PM, David Hildenbrand wrote: > On 21.11.22 16:00, Muhammad Usama Anjum wrote: >> 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. > > Quick reproducer (the first and easiest one that triggered :) ) > attached. > > With no kernel changes, it works as expected. > > # ./softdirty_mprotect > > > With the following kernel change to simulate what you propose it fails: > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index d22687d2e81e..f2c682bf7f64 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1457,8 +1457,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct > pagemapread *pm, > flags |= PM_FILE; > if (page && !migration && page_mapcount(page) == 1) > flags |= PM_MMAP_EXCLUSIVE; > - if (vma->vm_flags & VM_SOFTDIRTY) > - flags |= PM_SOFT_DIRTY; > + //if (vma->vm_flags & VM_SOFTDIRTY) > + // flags |= PM_SOFT_DIRTY; > > return make_pme(frame, flags); > } > > > # ./softdirty_mprotect > Page #1 should be softdirty > Thank you so much for sharing the issue and reproducer. After remapping the second part of the memory and m-protecting + m-unprotecting the whole memory, the PTE of the first half of the memory doesn't get marked as soft dirty even after writing multiple times to it. Even if soft-dirtiness is cleared on the whole process, the PTE of the first half memory doesn't get dirty. This seems like more of a bug in mprotect. The mprotect should not mess up with the soft-dirty flag in the PTEs. I'm debugging this. I hope to find the issue soon. Soft-dirty tracking in PTEs should be working correctly irrespective of the VM_SOFTDIRTY is set or not on the VMA. Cyrill has said in [1]: > ioctl might be an option indeed It brings some hope to this patch-set. [1] https://lore.kernel.org/all/Y4W0axw0ZgORtfkt@grain/ -- BR, Muhammad Usama Anjum
