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

[Suren Baghdasaryan]

On Wed, Jul 5, 2023 at 12:06 PM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jul 05, 2023 at 01:33:48PM -0400, Liam R. Howlett wrote:
> > * 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.
>
> 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.

Jumping into the conversation.

If we are adding a VMA into the tree before it's fully usable then we
should write-lock it before it becomes visible to page faults. Kirill
is right that there is a problem and we should not rely on
vma->vm_file->f_mapping lock here. Instead we should write-lock the
VMA before adding it into the tree even without this change.
IIUC, the rule with mmap_lock is that VMA can be safely modified if it
is either isolated or if we write-locked the mmap_lock. With
CONFIG_PER_VMA_LOCK the same rule should apply to per-VMA locks - the
VMA should be either isolated or we should write-lock it. Here we are
adding the unlocked VMA into the tree and then we keep modifying it.
This did not bite us because modifications are only done to
file-backed VMAs and we do not handle file-backed page faults under
per-VMA locks yet, however this will become a problem once we start
supporting that.
If we all agree to the above I can post a change to lock the VMA
before adding it into the tree.

>
> > 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.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov
>