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+<think>
+Okay, so I'm trying to help figure out why there's this issue happening during an rsync in a QEMU/KVM VM. Let me try to unpack the problem step by step.
+
+First, looking at the provided information, it seems that when performing an rsync from an NTFS partition (mounted as /mnt/b) to an ext4 partition (mounted as /mnt/a), there's some kind of bug occurring in the guest kernel. The user has already gone through several steps to reproduce this, and they've noticed that the issue only arises under specific conditions involving a dynamically compressed QCOW2 image.
+
+They mentioned that when using different setups—like raw images, fixed Qcow2, or even directly on the host without the VM—the bug doesn't occur. The problem specifically occurs with a dynamically compressed QCow2 image paired with an ext4 filesystem in the guest.
+
+Looking at their test cases:
+
+- It works with ExFAT and Raw/Qcow2 (fixed), but not with dynamic Qcow2.
+- It also works when using NTFS3 or other setups, except for this particular combination.
+
+So, the bug is triggered by a dynamically compressed QCow2 image combined with an ext4 filesystem in the guest. The user has also filed a kernel bug report related to memory allocation during rsync, which points towards some issue in how memory is being managed in the VM.
+
+I need to think about possible causes here. Since it's happening during an rsync operation, which involves copying data between two filesystems, maybe there's something wrong with how the block layer or storage devices are handled in QEMU when using dynamic compression.
+
+Dynamic compression in QCow2 can lead to fragmentation and possibly higher latency because the image needs to be expanded as needed. This might affect performance during heavy I/O operations like rsync, which is transferring a significant amount of data.
+
+Another angle is that the ext4 filesystem in the guest might not handle certain block allocations efficiently when backed by a dynamically compressed QCow2 image, leading to issues with memory charges or page allocation errors.
+
+The error mentioned in dmesg about memcg_kmem_charge_page and alloc_pages suggests that there's some memory pressure or failure in allocating pages. This could be due to how the memory controller groups are set up for the VM, but since it only happens in this specific setup, maybe it's related to how QEMU handles storage I/O in conjunction with dynamic images.
+
+I should also consider whether the block driver stack is correctly handling the QCow2 format, especially in a dynamically compressed scenario. Perhaps there's an issue where certain operations aren't being properly accounted for, leading to memory allocation problems during high I/O.
+
+The user has already pointed out that this doesn't happen on the host directly or with other image types, which implies it's something specific about how QEMU/KVM interacts with dynamic QCow2 images and ext4 in a VM. Maybe there's some misconfiguration or missing optimization in how the storage devices are presented to the guest.
+
+In terms of troubleshooting steps, perhaps adjusting certain QEMU parameters related to memory ballooning, I/O throughput, or block device caching could help. Alternatively, checking for any known issues with dynamic QCow2 images and ext4 in recent kernel versions might be necessary.
+
+Since they've already filed a bug report, it's possible that this is an upstream issue that needs patching in the Linux kernel or QEMU itself. In the meantime, working around the problem by using fixed-size QCow2 images or switching to raw images might be a viable solution for the user until a proper fix is available.
+
+I think the key takeaway here is that the combination of dynamic compression and ext4 in the VM's storage setup leads to memory allocation issues during data transfer. This suggests that the way QEMU handles I/O with dynamically compressed images isn't fully optimized or has a bug when paired with certain filesystems.
+</think>
+
+The issue arises due to specific interactions between a dynamically compressed QCow2 image, an ext4 filesystem in the guest VM, and the rsync operation. The combination leads to memory allocation problems, likely due to how QEMU/KVM handles I/O with dynamic images.
+
+**Root Cause:**
+- **Dynamic Compression Issue:** Dynamically compressed QCow2 images can cause higher latency and fragmentation during I/O operations, affecting performance.
+- **Memory Management Bug:** During rsync, the kernel encounters difficulties in page allocation (memcg_kmem_charge_page), suggesting a memory management issue specific to this setup.
+
+**Recommendations:**
+1. **Workaround:** Use fixed-size QCow2 images or raw images instead of dynamically compressed ones for more stable I/O handling.
+2. **Configuration Adjustments:** Optimize QEMU parameters related to storage and I/O, such as caching modes or memory ballooning, to alleviate memory pressure during transfers.
+3. **Bug Reporting/Tracking:** Monitor the upstream bug report (Linux kernel ticket #215563) for updates and potential patches.
+
+By avoiding dynamically compressed images, the user can prevent the issue until a definitive fix is implemented in QEMU/KVM or the Linux kernel.
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