Hi Andrew, On 07/19/18 at 12:44pm, Andrew Morton wrote: > On Thu, 19 Jul 2018 23:17:53 +0800 Baoquan He <bhe@xxxxxxxxxx> wrote: > > > 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? Sorry for late reply because of some urgent customer hotplug issues. I am rewriting all change logs, and cover letter. Then found I was wrong about the 2nd reason. The current kexec_file_load calls kexec_locate_mem_hole() to go through all system RAM region, if one region is larger than the size of kernel or initrd, it will search a position in that region from top to down. Since kexec will jump to 2nd kernel and don't need to care the 1st kernel's data, we can always find a usable space to load kexec kernel/initrd under 4G. So the only reason for this patch is keeping consistent with kexec_load and avoid confusion. And since x86 5-level paging mode has been added, we have another issue for top-down searching in the whole system RAM. That is we support dynamic 4-level to 5-level changing. Namely a kernel compiled with 5-level support, we can add 'no5lvl' to force 4-level. Then jumping from a 5-level kernel to 4-level kernel, e.g we load kernel at the top of system RAM in 5-level paging mode which might be bigger than 64TB, then try to jump to 4-level kernel with the upper limit of 64TB. For this case, we need add limit for kexec kernel loading if in 5-level kernel. All this mess makes me hesitate to choose a deligate method. Maybe I should drop this patchset. > > > > > > > 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 ;) For this issue, if we stop changing the kexec top down searching code, I am not sure if we should post this replacing with list_head patches separately. Thanks Baoquan
