On 02/13/16 at 08:57pm, Lasse Collin wrote: > On 2016-02-12 Baoquan He wrote: > > Now I have a question about the commit from you: > > > > commit 303148045aac34b70db722a54e5ad94a3a6625c6 > > Author: Lasse Collin <lasse.col...@tukaani.org> > > Date: Wed Jan 12 17:01:24 2011 -0800 > > > > x86: support XZ-compressed kernel > > > > > > In this commit for adding support of XZ-compressed kernel on x86, you > > add extra 32K to the extract_offset. In commit log you said this is > > because "The XZ decompressor needs around 30 KiB of heap, so the heap > > size is increased to 32 KiB on both x86-32 and x86-64." With my > > understanding decompression is done in decompression stage and it uses > > boot_heap in arch/x86/boot/compressed/head_64.S, and boot_heap is > > assigned to free_mem_ptr which is used for decompression heap malloc. > > During this decompressio stage it's still in copied ZO space, why did > > you add extra 32K space to extract_offset? If you want to increase > > the decompression heap space shouldn't you decrease the > > extract_offset? Do I misunderstand anything or miss things? > > The reason to increase the heap size in arch/x86/include/asm/boot.h is > unrelated to the reason why the offset was changed in > arch/x86/boot/compressed/mkpiggy.c. > > The long comment in arch/x86/boot/compressed/misc.c explains the need > for the offset for gzip/Deflate. A similar comment in > lib/decompress_unxz.c explains it for XZ/LZMA2.
Thank you so much, Lasse. You clearly pointed out my confusion. Yeah, I didn't understand it well. Your description for xz in lib/decompress_unxz.c is very helpful. The 64K is the maximum payload in one chunk. Adding this 64K is to avoid the worst case that very small payload can reprsent a 64K uncompressed output data. With my understanding it could be a chunk which contains complete duplicate content. like all "0" or other stuff? Thanks Baoquan > > Smaller safety-margins can work in practice since the calculated > margins are for the worst case. I'm not even sure if such calculations > have been done for the other decompressors in Linux.
