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

[Liam R. Howlett]

* Yu Ma <yu.ma@xxxxxxxxx> [230606 08:23]:
> 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 v6.4-rc4 due to regression of this benchmark
> from this commit f1a7941243c102a44e8847e3b94ff4ff3ec56f25 (mm: convert 
> mm's rss stats into percpu_counter).

I didn't think it was safe to insert a VMA into the VMA tree without
holding this write lock?  We now have a window of time where a file
mapping doesn't exist for a vma that's in the tree?  Is this always
safe?  Does the locking order in mm/rmap.c need to change?

>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@xxxxxxxxx/

I don't see a conclusion on that thread talking about changing the
locking order?

> 
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> ---
>  mm/mmap.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 13678edaa22c..0e694a0433bc 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2711,12 +2711,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	if (vma_iter_prealloc(&vmi))
>  		goto close_and_free_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);
>  
> -- 
> 2.39.3
> 
>