On Thu, 19 Jul 2018 23:17:53 +0800 Baoquan He <bhe@xxxxxxxxxx> wrote: > Hi Andrew, > > On 07/18/18 at 03:33pm, Andrew Morton wrote: > > On Wed, 18 Jul 2018 10:49:44 +0800 Baoquan He <bhe@xxxxxxxxxx> wrote: > > > > > For kexec_file loading, if kexec_buf.top_down is 'true', the memory which > > > is used to load kernel/initrd/purgatory is supposed to be allocated from > > > top to down. This is what we have been doing all along in the old kexec > > > loading interface and the kexec loading is still default setting in some > > > distributions. However, the current kexec_file loading interface doesn't > > > do like this. The function arch_kexec_walk_mem() it calls ignores checking > > > kexec_buf.top_down, but calls walk_system_ram_res() directly to go through > > > all resources of System RAM from bottom to up, to try to find memory region > > > which can contain the specific kexec buffer, then call locate_mem_hole_callback() > > > to allocate memory in that found memory region from top to down. This brings > > > confusion especially when KASLR is widely supported , users have to make clear > > > why kexec/kdump kernel loading position is different between these two > > > interfaces in order to exclude unnecessary noises. Hence these two interfaces > > > need be unified on behaviour. > > > > As far as I can tell, the above is the whole reason for the patchset, > > yes? To avoid confusing users. > > > In fact, it's not just trying to avoid confusing users. Kexec loading > and kexec_file loading are just do the same thing in essence. Just we > need do kernel image verification on uefi system, have to port kexec > loading code to kernel. > > Kexec has been a formal feature in our distro, and customers owning > those kind of very large machine can make use of this feature to speed > up the reboot process. On uefi machine, the kexec_file loading will > search place to put kernel under 4G from top to down. As we know, the > 1st 4G space is DMA32 ZONE, dma, pci mmcfg, bios etc all try to consume > it. It may have possibility to not be able to find a usable space for > kernel/initrd. From the top down of the whole memory space, we don't > have this worry. > > And at the first post, I just posted below with AKASHI's > walk_system_ram_res_rev() version. Later you suggested to use > list_head to link child sibling of resource, see what the code change > looks like. > http://lkml.kernel.org/r/20180322033722.9279-1-bhe@xxxxxxxxxx > > Then I posted v2 > http://lkml.kernel.org/r/20180408024724.16812-1-bhe@xxxxxxxxxx > Rob Herring mentioned that other components which has this tree struct > have planned to do the same thing, replacing the singly linked list with > list_head to link resource child sibling. Just quote Rob's words as > below. I think this could be another reason. > > ~~~~~ From Rob > The DT struct device_node also has the same tree structure with > parent, child, sibling pointers and converting to list_head had been > on the todo list for a while. ACPI also has some tree walking > functions (drivers/acpi/acpica/pstree.c). Perhaps there should be a > common tree struct and helpers defined either on top of list_head or a > ~~~~~ > new struct if that saves some size. Please let's get all this into the changelogs? > > > > Is that sufficient? Can we instead simplify their lives by providing > > better documentation or informative printks or better Kconfig text, > > etc? > > > > And who *are* the people who are performing this configuration? Random > > system administrators? Linux distro engineers? If the latter then > > they presumably aren't easily confused! > > Kexec was invented for kernel developer to speed up their kernel > rebooting. Now high end sever admin, kernel developer and QE are also > keen to use it to reboot large box for faster feature testing, bug > debugging. Kernel dev could know this well, about kernel loading > position, admin or QE might not be aware of it very well. > > > > > In other words, I'm trying to understand how much benefit this patchset > > will provide to our users as a whole. > > Understood. The list_head replacing patch truly involes too many code > changes, it's risky. I am willing to try any idea from reviewers, won't > persuit they have to be accepted finally. If don't have a try, we don't > know what it looks like, and what impact it may have. I am fine to take > AKASHI's simple version of walk_system_ram_res_rev() to lower risk, even > though it could be a little bit low efficient. The larger patch produces a better result. We can handle it ;)
