Ronit halder <ronit.crj@xxxxxxxxx> writes: > Currently, linux kernel allocates memory at the boot time for crash > kernel booted using kexec system call in kernel crash. It will be very > useful if we can allocate memory in run time. The user can get more > usable RAM if we can do that. > > It is possible to allocate memory for crash kernel at the boot time > using CMA (Contiguous Memory Allocator). CMA is capable of allocating > big chunk of memory. At the boot time we will create one (if only low > memory is used) or two (if we use both high and low memory) CMA areas > of size given in "crashkernel" boot time command line parameter. This > memory in CMA areas can be used by movable pages (used for disk caches, > process pages, etc) if not allocated. Then the user can allocate or > free memory from those CMA areas using "/sys/kernel/kexec_crash_size" > sysfs entry. If the user uses high memory kernel will automatically at > least 256MB low memory when the user allocates memory using mentioned > sysfs enrty. In case of high memory allocation the user controls the > size of reserved region in high memory with > "/sys/kernel/kexec_crash_size" entry. If the size set is zero then the > memory allocated in low memory will automatically be freed. > > I am attaching a prototype patch with the mail. Please share your > opinions on my approach. This patch is only for x86 and x86_64. > Please note, this patch is only a prototype just to explain my approach > and get the review. > > Signed-off-by: Ronit Halder <ronit.crj@xxxxxxxxx> So basically no. The reason for allocating memory at boot time is to guarantee that the kernel never uses that memory for anything and thus cannont get confused and use a stale DMA address and DMA onto that memory. Also since the memory has always been reserved the memory is likely not adjacent to any address used for DMA either. Nothing you have said even addresses the reason for allocating memory the way we do. If we wanted efficiency we would allocating the memory a page at a time the way kexec does. So I don't see anything that even tempts me to evaluate this patch. Eric _______________________________________________ kernel mailing list kernel@xxxxxxxxxxxxxxxxxxxxxxx https://lists.fedoraproject.org/admin/lists/kernel@xxxxxxxxxxxxxxxxxxxxxxx
