mm/mmap: move vma operations to mm_struct out of the critical section of file mapping lock

UnixBench/Execl represents a class of workload where bash scripts are
spawned frequently to do some short jobs.  When running multiple parallel
tasks, hot osq_lock is observed from do_mmap and exit_mmap.  Both of them
come from load_elf_binary through the call chain
"execl->do_execveat_common->bprm_execve->load_elf_binary".

In do_mmap,it will call mmap_region to create vma node, initialize it and
insert it to vma maintain structure in mm_struct and i_mmap tree of the
mapping file, then increase map_count to record the number of vma nodes
used.  The hot osq_lock is to protect operations on file's i_mmap tree. 
For the mm_struct member change like vma insertion and map_count update,
they do not affect i_mmap tree.  Move those operations out of the lock's
critical section, to reduce hold time on the lock.

With this change, on Intel Sapphire Rapids 112C/224T platform, based on
v6.0-rc6, the 160 parallel score improves by 12%.  The patch has no
obvious performance gain on v6.5-rc1 due to regression of this benchmark
from this commit f1a79412 (mm: convert
mm's rss stats into percpu_counter).  Related discussion and conclusion
can be referred at the mail thread initiated by 0day as below: Link:
https://lore.kernel.org/linux-mm/a4aa2e13-7187-600b-c628-7e8fb108def0@intel.com/

Link: https://lkml.kernel.org/r/20230712145739.604215-1-yu.ma@intel.comSigned-off-by: Yu Ma <yu.ma@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Kirill A . Shutemov <kirill@shutemov.name>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Zhu, Lipeng <lipeng.zhu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -412,14 +412,11 @@ static int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
 	if (vma_iter_prealloc(&vmi))
 		return -ENOMEM;
 
+	vma_iter_store(&vmi, vma);
+
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
 		i_mmap_lock_write(mapping);
-	}
-
-	vma_iter_store(&vmi, vma);
-
-	if (mapping) {
 		__vma_link_file(vma, mapping);
 		i_mmap_unlock_write(mapping);
 	}
@@ -2812,12 +2809,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 	/* Lock the VMA since it is modified after insertion into VMA tree */
 	vma_start_write(vma);
-	if (vma->vm_file)
-		i_mmap_lock_write(vma->vm_file->f_mapping);
-
 	vma_iter_store(&vmi, vma);
 	mm->map_count++;
 	if (vma->vm_file) {
+		i_mmap_lock_write(vma->vm_file->f_mapping);
 		if (vma->vm_flags & VM_SHARED)
 			mapping_allow_writable(vma->vm_file->f_mapping);