Re: [PATCH v8 0/3] Poisoned memory recovery on reboot

[William Roche <william.roche@oracle.com>]

On 2/11/25 23:35, Peter Xu wrote:
> On Tue, Feb 11, 2025 at 09:27:04PM +0000, “William Roche wrote:
>> From: William Roche <william.roche@oracle.com>
>>
>> Here is a very simplified version of my fix only dealing with the
>> recovery of huge pages on VM reset.
>>   ---
>> This set of patches fixes an existing bug with hardware memory errors
>> impacting hugetlbfs memory backed VMs and its recovery on VM reset.
>> When using hugetlbfs large pages, any large page location being impacted
>> by an HW memory error results in poisoning the entire page, suddenly
>> making a large chunk of the VM memory unusable.
>>
>> The main problem that currently exists in Qemu is the lack of backend
>> file repair before resetting the VM memory, resulting in the impacted
>> memory to be silently unusable even after a VM reboot.
>>
>> In order to fix this issue, we take into account the page size of the
>> impacted memory block when dealing with the associated poisoned page
>> location.
>>
>> Using the page size information we also try to regenerate the memory
>> calling ram_block_discard_range() on VM reset when running
>> qemu_ram_remap(). So that a poisoned memory backed by a hugetlbfs
>> file is regenerated with a hole punched in this file. A new page is
>> loaded when the location is first touched.  In case of a discard
>> failure we fall back to remapping the memory location.
>>
>> But we currently don't reset the memory settings and the 'prealloc'
>> attribute is ignored after the remap from the file backend.
> 
> queued patch 1-2, thanks.
> 

Thank you very much Peter, and thanks to David too !

According to me, ARM needs more than only error injection messages.
For example, the loop of errors that can appear during kdump when 
dealing with large pages is a real problem, hanging a VM.

There is also the remap notification (to better deal with 'prealloc' 
attribute for example) that needs to be implemented now.

And finally the kernel KVM enhancement needed on x86 to return a more 
accurate SIGBUS siginfo.si_addr_lsb value on large pages memory errors.
Qemu could than take this information into account to provide more 
useful feedback about the 'failed' memory size.

I don't know yet when I'll have the possibility to come back to these 
problems, but at least we have the recovery of large pages mostly fixed 
with the 2 patches queued.

Thanks again,
William.