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

[Liam R. Howlett]

* Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> [230705 12:54]:
> On Tue, Jun 06, 2023 at 03:20:13PM -0400, Liam R. Howlett wrote:
> > * 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?
> 
> We hold mmap lock on write here, right?

Yes.

>Who can observe the VMA until the
> lock is released?

With CONFIG_PER_VMA_LOCK we can have the VMA read under the rcu
read lock for page faults from the tree.  I am not sure if the vma is
initialized to avoid page fault issues - vma_start_write() should either
be taken or initialise the vma as this is the case.

There is also a possibility of a driver mapping a VMA and having entry
points from other locations.  It isn't accessed through the tree though
so I don't think this change will introduce new races?

> 
> It cannot be retrieved from the VMA tree as it requires at least read mmap
> lock. And the VMA doesn't exist anywhere else.
> 
> I believe the change is safe.

I guess insert_vm_struct(), and vma_link() callers should be checked and
updated accordingly?

Thanks,
Liam