From 64bd72048a2ac07efed70debe606a1c6e5e03554 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Fri, 31 Mar 2017 18:31:48 +0100 Subject: [PATCH] statx: optimize copy of struct statx to userspace I found that statx() was significantly slower than stat(). As a microbenchmark, I compared 10,000,000 invocations of fstat() on a tmpfs file to the same with statx() passed a NULL path: $ time ./stat_benchmark real 0m1.464s user 0m0.275s sys 0m1.187s $ time ./statx_benchmark real 0m5.530s user 0m0.281s sys 0m5.247s statx is expected to be a little slower than stat because struct statx is larger than struct stat, but not by *that* much. It turns out that most of the overhead was in copying struct statx to userspace, mostly in all the stac/clac instructions that got generated for each __put_user() call. (This was on x86_64, but some other architectures, e.g. arm64, have something similar now too.) stat() instead initializes its struct on the stack and copies it to userspace with a single call to copy_to_user(). This turns out to be much faster, and changing statx to do this makes it almost as fast as stat: $ time ./statx_benchmark real 0m1.624s user 0m0.270s sys 0m1.354s For zeroing the reserved fields, start by zeroing the full struct with memset. This makes it clear that every byte copied to userspace is initialized, even implicit padding bytes (though there are none currently). In the scenarios I tested, it also performed the same as a designated initializer. Manually initializing each field was still slightly faster, but would have been more error-prone and less verifiable. Also rename statx_set_result() to cp_statx() for consistency with cp_old_stat() et al., and make it noinline so that struct statx doesn't add to the stack usage during the main portion of the syscall execution. Signed-off-by: Eric Biggers Signed-off-by: David Howells Signed-off-by: Al Viro --- fs/stat.c | 74 +++++++++++++++++++++++++++------------------------------------ 1 file changed, 32 insertions(+), 42 deletions(-) diff --git a/fs/stat.c b/fs/stat.c index b792dd2..ab27f28 100644 --- a/fs/stat.c +++ b/fs/stat.c @@ -510,46 +510,37 @@ SYSCALL_DEFINE4(fstatat64, int, dfd, const char __user *, filename, } #endif /* __ARCH_WANT_STAT64 || __ARCH_WANT_COMPAT_STAT64 */ -static inline int __put_timestamp(struct timespec *kts, - struct statx_timestamp __user *uts) +static noinline_for_stack int +cp_statx(const struct kstat *stat, struct statx __user *buffer) { - return (__put_user(kts->tv_sec, &uts->tv_sec ) || - __put_user(kts->tv_nsec, &uts->tv_nsec ) || - __put_user(0, &uts->__reserved )); -} - -/* - * Set the statx results. - */ -static long statx_set_result(struct kstat *stat, struct statx __user *buffer) -{ - uid_t uid = from_kuid_munged(current_user_ns(), stat->uid); - gid_t gid = from_kgid_munged(current_user_ns(), stat->gid); - - if (__put_user(stat->result_mask, &buffer->stx_mask ) || - __put_user(stat->mode, &buffer->stx_mode ) || - __clear_user(&buffer->__spare0, sizeof(buffer->__spare0)) || - __put_user(stat->nlink, &buffer->stx_nlink ) || - __put_user(uid, &buffer->stx_uid ) || - __put_user(gid, &buffer->stx_gid ) || - __put_user(stat->attributes, &buffer->stx_attributes ) || - __put_user(stat->blksize, &buffer->stx_blksize ) || - __put_user(MAJOR(stat->rdev), &buffer->stx_rdev_major ) || - __put_user(MINOR(stat->rdev), &buffer->stx_rdev_minor ) || - __put_user(MAJOR(stat->dev), &buffer->stx_dev_major ) || - __put_user(MINOR(stat->dev), &buffer->stx_dev_minor ) || - __put_timestamp(&stat->atime, &buffer->stx_atime ) || - __put_timestamp(&stat->btime, &buffer->stx_btime ) || - __put_timestamp(&stat->ctime, &buffer->stx_ctime ) || - __put_timestamp(&stat->mtime, &buffer->stx_mtime ) || - __put_user(stat->ino, &buffer->stx_ino ) || - __put_user(stat->size, &buffer->stx_size ) || - __put_user(stat->blocks, &buffer->stx_blocks ) || - __clear_user(&buffer->__spare1, sizeof(buffer->__spare1)) || - __clear_user(&buffer->__spare2, sizeof(buffer->__spare2))) - return -EFAULT; - - return 0; + struct statx tmp; + + memset(&tmp, 0, sizeof(tmp)); + + tmp.stx_mask = stat->result_mask; + tmp.stx_blksize = stat->blksize; + tmp.stx_attributes = stat->attributes; + tmp.stx_nlink = stat->nlink; + tmp.stx_uid = from_kuid_munged(current_user_ns(), stat->uid); + tmp.stx_gid = from_kgid_munged(current_user_ns(), stat->gid); + tmp.stx_mode = stat->mode; + tmp.stx_ino = stat->ino; + tmp.stx_size = stat->size; + tmp.stx_blocks = stat->blocks; + tmp.stx_atime.tv_sec = stat->atime.tv_sec; + tmp.stx_atime.tv_nsec = stat->atime.tv_nsec; + tmp.stx_btime.tv_sec = stat->btime.tv_sec; + tmp.stx_btime.tv_nsec = stat->btime.tv_nsec; + tmp.stx_ctime.tv_sec = stat->ctime.tv_sec; + tmp.stx_ctime.tv_nsec = stat->ctime.tv_nsec; + tmp.stx_mtime.tv_sec = stat->mtime.tv_sec; + tmp.stx_mtime.tv_nsec = stat->mtime.tv_nsec; + tmp.stx_rdev_major = MAJOR(stat->rdev); + tmp.stx_rdev_minor = MINOR(stat->rdev); + tmp.stx_dev_major = MAJOR(stat->dev); + tmp.stx_dev_minor = MINOR(stat->dev); + + return copy_to_user(buffer, &tmp, sizeof(tmp)) ? -EFAULT : 0; } /** @@ -573,8 +564,6 @@ SYSCALL_DEFINE5(statx, if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE) return -EINVAL; - if (!access_ok(VERIFY_WRITE, buffer, sizeof(*buffer))) - return -EFAULT; if (filename) error = vfs_statx(dfd, filename, flags, &stat, mask); @@ -582,7 +571,8 @@ SYSCALL_DEFINE5(statx, error = vfs_statx_fd(dfd, &stat, mask, flags); if (error) return error; - return statx_set_result(&stat, buffer); + + return cp_statx(&stat, buffer); } /* Caller is here responsible for sufficient locking (ie. inode->i_lock) */ -- 2.7.4