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Jeff Moyer reported that on his system with two memory regions 0~64G and 1T~1T+192G, and kernel option "memmap=192G!1024G" added, enabling KASLR will make the system hang intermittently during boot. While adding 'nokaslr' won't. The back trace is: Oops: 0000 [#1] SMP RIP: memcpy_erms() [ .... ] Call Trace: pmem_rw_page() bdev_read_page() do_mpage_readpage() mpage_readpages() blkdev_readpages() __do_page_cache_readahead() force_page_cache_readahead() page_cache_sync_readahead() generic_file_read_iter() blkdev_read_iter() __vfs_read() vfs_read() SyS_read() entry_SYSCALL_64_fastpath() This crash happens because the for loop count calculation in sync_global_pgds() is not correct. When a mapping area crosses PGD entries, we should calculate the starting address of region which next PGD covers and assign it to next for loop count, but not add PGDIR_SIZE directly. The old code works right only if the mapping area is an exact multiple of PGDIR_SIZE, otherwize the end region could be skipped so that it can't be synchronized to all other processes from kernel PGD init_mm.pgd. In Jeff's system, emulated pmem area [1024G, 1216G) is smaller than PGDIR_SIZE. While 'nokaslr' works because PAGE_OFFSET is 1T aligned, it makes this area be mapped inside one PGD entry. With KASLR enabled, this area could cross two PGD entries, then the next PGD entry won't be synced to all other processes. That is why we saw empty PGD. Fix it. Reported-by: Jeff Moyer <jmoyer@redhat.com> Signed-off-by: Baoquan He <bhe@redhat.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Dave Young <dyoung@redhat.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jinbum Park <jinb.park7@gmail.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Kees Cook <keescook@chromium.org> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Garnier <thgarnie@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Yasuaki Ishimatsu <yasu.isimatu@gmail.com> Cc: Yinghai Lu <yinghai@kernel.org> Link: http://lkml.kernel.org/r/1493864747-8506-1-git-send-email-bhe@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.