On 16.07.21 г. 1:33, Dave Chinner wrote: > On Thu, Jul 15, 2021 at 04:09:06PM +0100, Matthew Wilcox wrote: >> On Thu, Jul 15, 2021 at 05:44:15PM +0300, Nikolay Borisov wrote: >>> 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. > > Yup, this. memcmp() is widley used in hot paths through all the > filesystem code and the rest of the kernel. We should fix the > generic infrastructure problem, not play whack-a-mole to with custom > one-off fixes that avoid the problem just where it shows up in some > profile... I ported glibc's implementation of memcmp to the kernel and after running the same workload I get the same performance as with the basic memcmp implementation of doing byte comparison ... > > Cheers, > > Dave. >
