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

[Liam R. Howlett]

* Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [230606 15:20]:
> * 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?

So I'm pretty sure it's not safe because we've been ensuring that this
lock was taken during vma tree inserts since 2002 [1].  Take a look at
vma_link() in that commit.  I still don't have an answer as to why it's
not safe though.

[1] https://github.com/mpe/linux-fullhistory/commit/bbbce8f41d3da0ac968bab7a967e12e2be1a7eb0

> 
> >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
> > 
> >