Re: [PATCH v7 3/6] accel/kvm: Report the loss of a large memory page

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 05, 2025 at 05:27:13PM +0100, William Roche wrote:
> On 2/4/25 18:01, Peter Xu wrote:
> > On Sat, Feb 01, 2025 at 09:57:23AM +0000, “William Roche wrote:
> > > From: William Roche <william.roche@oracle.com>
> > > 
> > > In case of a large page impacted by a memory error, provide an
> > > information about the impacted large page before the memory
> > > error injection message.
> > > 
> > > This message would also appear on ras enabled ARM platforms, with
> > > the introduction of an x86 similar error injection message.
> > > 
> > > In the case of a large page impacted, we now report:
> > > Memory Error on large page from <backend>:<address>+<fd_offset> +<page_size>
> > > 
> > > The +<fd_offset> information is only provided with a file backend.
> > > 
> > > Signed-off-by: William Roche <william.roche@oracle.com>
> > 
> > This is still pretty kvm / arch relevant patch that needs some reviews.
> > 
> > I wonder do we really need this - we could fetch ramblock mapping
> > (e.g. hwaddr -> HVA) via HMP "info ramblock", and also dmesg shows process
> > ID + VA.  IIUC we have all below info already as long as we do some math
> > based on above.  Would that work too?
> 
> The HMP command "info ramblock" is implemented with the ram_block_format()
> function which returns a message buffer built with a string for each
> ramblock (protected by the RCU_READ_LOCK_GUARD). Our new function copies a
> struct with the necessary information.
> 
> Relaying on the buffer format to retrieve the information doesn't seem
> reasonable, and more importantly, this buffer doesn't provide all the needed
> data, like fd and fd_offset.
> 
> I would say that ram_block_format() and qemu_ram_block_info_from_addr()
> serve 2 different goals.
> 
> (a reimplementation of ram_block_format() with an adapted version of
> qemu_ram_block_info_from_addr() taking the extra information needed could be
> doable for example, but may not be worth doing for now)

IIUC admin should be aware of fd_offset because the admin should be fully
aware of the start offset of FDs to specify in qemu cmdlines, or in
Libvirt. But yes, we can always add fd_offset into ram_block_format() if
it's helpful.

Besides, the existing issues on this patch:

  - From outcome of this patch, it introduces one ramblock API (which is ok
    to me, so far), to do some error_report()s.  It looks pretty much for
    debugging rather than something serious (e.g. report via QMP queries,
    QMP events etc.).  From debug POV, I still don't see why this is
    needed.. per discussed above.

  - From merge POV, this patch isn't a pure memory change, so I'll need to
    get ack from other maintainers, at least that should be how it works..

I feel like when hwpoison becomes a serious topic, we need some more
serious reporting facility than error reports.  So that we could have this
as separate topic to be revisited.  It might speed up your prior patches
from not being blocked on this.

Thanks,

-- 
Peter Xu