RE: [PATCH] mm/mmap: move vmKirill A. Shutemov <kirill@shutemov.name>a operations to mm_struct out of the critical section of file mapping lock

["Ma, Yu" <yu.ma@intel.com>]

> -----Original Message-----
> From: Kirill A. Shutemov <kirill@shutemov.name>
> Sent: Thursday, July 6, 2023 3:06 AM
> To: Liam R. Howlett <Liam.Howlett@Oracle.com>; Ma, Yu
> <yu.ma@intel.com>; akpm@linux-foundation.org; Chen, Tim C
> <tim.c.chen@intel.com>; linux-mm@kvack.org; linux-kernel@vger.kernel.org;
> Hansen, Dave <dave.hansen@intel.com>; Williams, Dan J
> <dan.j.williams@intel.com>; shakeelb@google.com; Deng, Pan
> <pan.deng@intel.com>; Li, Tianyou <tianyou.li@intel.com>; Zhu, Lipeng
> <lipeng.zhu@intel.com>; tim.c.chen@linux.intel.com
> Subject: Re: [PATCH] mm/mmap: move vma operations to mm_struct out of
> the critical section of file mapping lock
> 
> On Wed, Jul 05, 2023 at 01:33:48PM -0400, Liam R. Howlett wrote:
> > * Kirill A. Shutemov <kirill@shutemov.name> [230705 12:54]:
> > > On Tue, Jun 06, 2023 at 03:20:13PM -0400, Liam R. Howlett wrote:
> > > > * Yu Ma <yu.ma@intel.com> [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.
> 
> Right, with CONFIG_PER_VMA_LOCK the vma has to be unusable until it is
> fully initialized, effectively providing the same guarantees as mmap write lock.
> If it is not the case, it is CONFIG_PER_VMA_LOCK bug.
> 
> > 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?
> 
> Right.
> 
> > > 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?
> 
> Yep.
> 
Thanks Kirill and Liam, I'll update patch and send out for review.


Regards
Yu