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Dave Hansen authored
Summary: In current kernels, with PTI enabled, no pages are marked Global. This potentially increases TLB misses. But, the mechanism by which the Global bit is set and cleared is rather haphazard. This patch makes the process more explicit. In the end, it leaves us with Global entries in the page tables for the areas truly shared by userspace and kernel and increases TLB hit rates. The place this patch really shines in on systems without PCIDs. In this case, we are using an lseek microbenchmark[1] to see how a reasonably non-trivial syscall behaves. Higher is better: No Global pages (baseline): 6077741 lseeks/sec 88 Global Pages (this set): 7528609 lseeks/sec (+23.9%) On a modern Skylake desktop with PCIDs, the benefits are tangible, but not huge for a kernel compile (lower is better): No Global pages (baseline): 186.951 seconds time elapsed ( +- 0.35% ) 28 Global pages (this set): 185.756 seconds time elapsed ( +- 0.09% ) -1.195 seconds (-0.64%) I also re-checked everything using the lseek1 test[1]: No Global pages (baseline): 15783951 lseeks/sec 28 Global pages (this set): 16054688 lseeks/sec +270737 lseeks/sec (+1.71%) The effect is more visible, but still modest. Details: The kernel page tables are inherited from head_64.S which rudely marks them as _PAGE_GLOBAL. For PTI, we have been relying on the grace of $DEITY and some insane behavior in pageattr.c to clear _PAGE_GLOBAL. This patch tries to do better. First, stop filtering out "unsupported" bits from being cleared in the pageattr code. It's fine to filter out *setting* these bits but it is insane to keep us from clearing them. Then, *explicitly* go clear _PAGE_GLOBAL from the kernel identity map. Do not rely on pageattr to do it magically. After this patch, we can see that "GLB" shows up in each copy of the page tables, that we have the same number of global entries in each and that they are the *same* entries. /sys/kernel/debug/page_tables/current_kernel:11 /sys/kernel/debug/page_tables/current_user:11 /sys/kernel/debug/page_tables/kernel:11 9caae8ad6a1fb53aca2407ec037f612d current_kernel.GLB 9caae8ad6a1fb53aca2407ec037f612d current_user.GLB 9caae8ad6a1fb53aca2407ec037f612d kernel.GLB A quick visual audit also shows that all the entries make sense. 0xfffffe0000000000 is the cpu_entry_area and 0xffffffff81c00000 is the entry/exit text: 0xfffffe0000000000-0xfffffe0000002000 8K ro GLB NX pte 0xfffffe0000002000-0xfffffe0000003000 4K RW GLB NX pte 0xfffffe0000003000-0xfffffe0000006000 12K ro GLB NX pte 0xfffffe0000006000-0xfffffe0000007000 4K ro GLB x pte 0xfffffe0000007000-0xfffffe000000d000 24K RW GLB NX pte 0xfffffe000002d000-0xfffffe000002e000 4K ro GLB NX pte 0xfffffe000002e000-0xfffffe000002f000 4K RW GLB NX pte 0xfffffe000002f000-0xfffffe0000032000 12K ro GLB NX pte 0xfffffe0000032000-0xfffffe0000033000 4K ro GLB x pte 0xfffffe0000033000-0xfffffe0000039000 24K RW GLB NX pte 0xffffffff81c00000-0xffffffff81e00000 2M ro PSE GLB x pmd [1.] https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c Signed-off-by:Dave Hansen <dave.hansen@linux.intel.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: David Woodhouse <dwmw2@infradead.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Hugh Dickins <hughd@google.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Kees Cook <keescook@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Nadav Amit <namit@vmware.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-mm@kvack.org Link: http://lkml.kernel.org/r/20180406205517.C80FBE05@viggo.jf.intel.com Signed-off-by:
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