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

[Ma, Yu]

> * 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
> 

Thanks Liam for your quick review and digging in related code. I just checked vma_link() in 2002, the file lock is there to protect __vma_link(), and in __vma_link(), there are 3 functions, the first 2 are operations to insert vma to mm_struct, and the last func __vma_link_file() is to insert vma to the file mapping tree. So this file lock around __vma_link() makes sense since it has operations of file mapping tree inside. It still cannot explain why the operations to mm_struct cannot be moved out.

> >
> > >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/
> >
> > I don't see a conclusion on that thread talking about changing the
> > locking order?
I may not intro this link clear, it is about why no obvious improvement observed on latest kernel for the time being :)

> >
> > >
> > > 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
> > >
> > >