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envelope-from=william.roche@oracle.com; helo=mx0a-00069f02.pphosted.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2/5/25 18:07, Peter Xu wrote: > On Wed, Feb 05, 2025 at 05:27:13PM +0100, William Roche wrote: >> [...] >> 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. The reason why I want to inform the user of a large memory failure more specifically than a standard sized page loss is because of the significant behavior difference: Our current implementation can transparently handle many situations without necessarily leading the VM to a crash. But when it comes to large pages, there is no mechanism to inform the VM of a large memory loss, and usually this situation leads the VM to crash, and can also generate some weird situations like qemu itself crashing or a loop of errors, for example. So having a message informing of such a memory loss can help to understand a more radical VM or qemu behavior -- it increases the diagnosability of our code. To verify that a SIGBUS appeared because of a large page loss, we currently need to verify the targeted memory block backend page_size. We should usually get this information from the SIGBUS siginfo data (with a si_addr_lsb field giving an indication of the page size) but a KVM weakness with a hardcoded si_addr_lsb=PAGE_SHIFT value in the SIGBUS siginfo returned from the kernel prevents that: See kvm_send_hwpoison_signal() function. So I first wrote a small API addition called qemu_ram_pagesize_from_addr() to retrieve only this page_size value from the impacted address; and later on, this function turned into the richer qemu_ram_block_info_from_addr() function to have the generated messages match the existing memory messages as rightly requested by David. So the main reason is a KVM "weakness" with kvm_send_hwpoison_signal(), and the second reason is to have richer error messages. > - 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 agree :) > > 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. I explained why I think that error messages are important, but I don't want to get blocked on fixing the hugepage memory recovery because of that. If you think that not displaying a specific message for large page loss can help to get the recovery fixed, than I can change my proposal to do so. Early next week, I'll send a simplified version of my first 3 patches without this specific messages and without the preallocation handling in all remap cases, so you can evaluate this possibility. Thank again for your feedback William.