On Tue, Feb 12, 2019 at 4:48 AM Dave Young <dyoung@xxxxxxxxxx> wrote: > > On 02/06/19 at 08:08pm, Dave Young wrote: > > On 02/05/19 at 09:15am, Borislav Petkov wrote: > > > On Mon, Feb 04, 2019 at 03:30:16PM -0700, Jerry Hoemann wrote: > > > > Is your objection only to the second fallback of allocating > > > > memory above >= 4GB? Or are you objecting to allocating from > > > > (896 .. 4GB) as well? > > > > > > My problem is why should the user need to specify high or low allocation > > > explicitly when we can handle all that in the kernel automatically. > > > > > > The presence of crashkernel= on the cmdline sure means that the user > > > wants to allocate memory for a second kernel. > > > > > > Now, if the requested allocation fails, we say: > > > > > > Error reserving crashkernel > > > > > > So, instead of saying that, we can *try* *again* and say > > > > > > Error reserving requested crashkernel at @..., attempting a high range. > > > > > > and run memblock_find_in_range() on the other regions which we deemed > > > are ok to allocate from. > > > > > > Why aren't we doing that by default instead of placing all those > > > different options in front of the user and expecting her/him to know > > > something about all those magic ranges? > > > > As we talked in another reply, for the >4G allocation we can not avoid > > the swiotlb issue, but if one request for 256M in high region and we > > allocate the low part automatically, it will eat more memory eg. 512M. > > > > But probably in case allacation failed in low region ,high is a must for kdump > > reservation, since no other choices perhaps we can make that as you said > > That is exactly what Pingfan is doing in this patch. > > Even we make it automatic in kernel, but we have to have some default > value for swiotlb in case crashkernel can not find a free region under 4G. > So this default value can not work for every use cases, people need > manually use crashkernel=,low and crashkernel=,high in case > crashkernel=X does not work. One can tune it for their use: > > 1) crashkernel=X reservation fails, likely the ,low default value is > still too big, one can shrink the value and manually try other value > 2) crashkernel=X reserve successfully on high memory and along with some > default low memory region. But the low region is not enough. In this > case one can increase the > > This should answer the question why ,high and ,low is still needed. > > But for above consumption 1), KASLR can still cause default ,low memory > failed to reserve. So I wonder if KASLR can skip the 0-896M if the > system memory is big enough. > A little fix about the comment. Refer to reserve_crashkernel_low(), low_base = memblock_find_in_range(0, 1ULL << 32, low_size, CRASH_ALIGN); So it should try 0~4G for ",low". And the default size for low is 256M. Given the limited memory region reserved by other component before crashkernel, we always can find a continuous chunk of 256M inside the fragmented [0,4G], which is split by initrd, KASLR. Thanks, Pingfan _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
