On Thu, Jul 15, 2021 at 05:44:15PM +0300, Nikolay Borisov wrote: > That was my first impression, here's the profile: > > │ Disassembly of section .text: > │ > │ ffffffff815c6f60 <memcmp>: > │ memcmp(): > │ test %rdx,%rdx > │ ↓ je 22 > │ xor %ecx,%ecx > │ ↓ jmp 12 > 49.32 │ 9: add $0x1,%rcx > 0.03 │ cmp %rcx,%rdx > 11.82 │ ↓ je 21 > 0.01 │12: movzbl (%rdi,%rcx,1),%eax > 38.19 │ movzbl (%rsi,%rcx,1),%r8d > 0.59 │ sub %r8d,%eax > 0.04 │ ↑ je 9 That looks like a byte loop to me ... > It's indeed on x86-64 and according to the sources it's using > __builtin_memcmp according to arch/x86/boot/string.h I think the 'boot' part of that path might indicate that it's not what's actually being used by the kernel. $ git grep __HAVE_ARCH_MEMCMP arch/arc/include/asm/string.h:#define __HAVE_ARCH_MEMCMP arch/arm64/include/asm/string.h:#define __HAVE_ARCH_MEMCMP arch/csky/abiv2/inc/abi/string.h:#define __HAVE_ARCH_MEMCMP arch/powerpc/include/asm/string.h:#define __HAVE_ARCH_MEMCMP arch/s390/include/asm/string.h:#define __HAVE_ARCH_MEMCMP /* arch function */ arch/s390/lib/string.c:#ifdef __HAVE_ARCH_MEMCMP arch/s390/purgatory/string.c:#define __HAVE_ARCH_MEMCMP /* arch function */ arch/sparc/include/asm/string.h:#define __HAVE_ARCH_MEMCMP include/linux/string.h:#ifndef __HAVE_ARCH_MEMCMP lib/string.c:#ifndef __HAVE_ARCH_MEMCMP So I think x86-64 is using the stupid one. > > Can this even happen? Surely we can only dedup on a block boundary and > > blocks are required to be a power of two and at least 512 bytes in size? > > I was wondering the same thing, but AFAICS it seems to be possible i.e > if userspace spaces bad offsets, while all kinds of internal fs > synchronization ops are going to be performed on aligned offsets, that > doesn't mean the original ones, passed from userspace are themselves > aligned explicitly. Ah, I thought it'd be failed before we got to this point. But honestly, I think x86-64 needs to be fixed to either use __builtin_memcmp() or to have a nicely written custom memcmp(). I tried to find the gcc implementation of __builtin_memcmp() on x86-64, but I can't.
