V Thu, 24 May 2018 11:34:05 -0500 ebiederm@xxxxxxxxxxxx (Eric W. Biederman) napsáno: > Petr Tesarik <ptesarik@xxxxxxx> writes: > > 2> On Thu, 24 May 2018 09:49:05 +0800 > > Dave Young <dyoung@xxxxxxxxxx> wrote: > > > >> Hi Petr, > >> > >> On 05/23/18 at 10:22pm, Petr Tesarik wrote: > >>[...] > >> > In short, if one size fits none, what good is it to hardcode that "one > >> > size" into the kernel image? > >> > >> I agreed with all the things that we can not know the exact memory > >> requirement for 100% use cases. But that does not means this is useless > >> it is still useful for common use cases of no special and memory hog > >> requirements as I mentioned in another reply it can simplify the kdump > >> deployment for those people who do not need the special setup. > > > > I still tend to disagree. This "common-case" reservation depends on > > things that are defined by user space. It surely does not make it > > easier to build a distribution kernel. Today, I get bug reports that > > the number calculated and added to the boot loader configuration by the > > installer is inaccurate. If I put a fixed number into a kernel config > > option, I will start getting bugs that this number is incorrect (for > > some systems). > > > >> For example, if this is a workstation I just want to break into a shell > >> to collect some panic info, then I just need a very minimal initrd, then > >> the Kconfig will work just fine. > > > > What is "a very minimal initrd"? Last time I had to make a significant > > adjustment to the estimation for openSUSE, this was caused by growing > > user-space requirements (systemd in this case, but I don't want to > > start flamewars on that topic, please). > > > > Anyway, if you want to improve the "common case", then look how IBM > > tries to solve it for firmware-assisted dump (fadump) on powerpc: > > > > https://patchwork.ozlabs.org/patch/905026/ > > > > The main idea is: > > > >> Instead of setting aside a significant chunk of memory nobody can use, > >> [...] reserve a significant chunk of memory that the kernel is prevented > >> from using [...], but applications are free to use it. > > > > That works great, because user space pages are filtered out in the > > common case, so they can be used freely by the panic kernel. > > They absolutely can not be used in the kdump case. > > The kdump requirement is that they are pages no-one initiates any I/O > to. To avoid the problem of devices doing DMA as the new kernel starts > and runs. Good point. This means that memory reserved for this purpose would also have to be excluded from allocations that may be eventually used for DMA transfers. > Secondarily to avoid problems with cpus that refused to halt. Let's face it - if some CPUs refused to halt, all bets are off. The code running on such a CPU can break many other things besides memory, most importantly, it may meddle with the HW registers of crucial devices in the system. To be less abstract, I have seen a failure to stop a CPU in the crashed kernel a few times, and the panic kernel could never successfully save anything; it always crashed at boot or a little bit later. Anyway, of course we would still have to keep the current method, because user pages are not always filtered. For example, a major SUSE account runs a database in user space and also inspects its data structures in case of a system crash. Petr T _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
