Re: [PATCH v3 13/16] mm/mmap: Avoid zeroing vma tree in mmap_region()

[Liam R. Howlett]

* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [240708 08:18]:
> On Thu, Jul 04, 2024 at 02:27:15PM GMT, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> >
> > Instead of zeroing the vma tree and then overwriting the area, let the
> > area be overwritten and then clean up the gathered vmas using
> > vms_complete_munmap_vmas().
> >
> > In the case of a driver mapping over existing vmas, the PTEs are cleared
> > using the helper vms_complete_pte_clear().
> >
> > Temporarily keep track of the number of pages that will be removed and
> > reduce the charged amount.
> >
> > This also drops the validate_mm() call in the vma_expand() function.
> > It is necessary to drop the validate as it would fail since the mm
> > map_count would be incorrect during a vma expansion, prior to the
> > cleanup from vms_complete_munmap_vmas().
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > ---
> >  mm/internal.h |  1 +
> >  mm/mmap.c     | 61 ++++++++++++++++++++++++++++++---------------------
> >  2 files changed, 37 insertions(+), 25 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 4c9f06669cc4..fae4a1bba732 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -1503,6 +1503,7 @@ struct vma_munmap_struct {
> >  	unsigned long stack_vm;
> >  	unsigned long data_vm;
> >  	bool unlock;			/* Unlock after the munmap */
> > +	bool cleared_ptes;		/* If the PTE are cleared already */
> >  };
> >
> >  void __meminit __init_single_page(struct page *page, unsigned long pfn,
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 5d458c5f080e..0c334eeae8cd 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -401,17 +401,21 @@ anon_vma_interval_tree_post_update_vma(struct vm_area_struct *vma)
> >  }
> >
> >  static unsigned long count_vma_pages_range(struct mm_struct *mm,
> > -		unsigned long addr, unsigned long end)
> > +		unsigned long addr, unsigned long end,
> > +		unsigned long *nr_accounted)
> >  {
> >  	VMA_ITERATOR(vmi, mm, addr);
> >  	struct vm_area_struct *vma;
> >  	unsigned long nr_pages = 0;
> >
> > +	*nr_accounted = 0;
> >  	for_each_vma_range(vmi, vma, end) {
> >  		unsigned long vm_start = max(addr, vma->vm_start);
> >  		unsigned long vm_end = min(end, vma->vm_end);
> >
> >  		nr_pages += PHYS_PFN(vm_end - vm_start);
> > +		if (vma->vm_flags & VM_ACCOUNT)
> > +			*nr_accounted += PHYS_PFN(vm_end - vm_start);
> 
> We're duplicating the PHYS_PFN(vm_end - vm_start) thing, probably worth
> adding something like:
> 
> unsigned long num_pages = PHYS_PFN(vm_end - vm_start);
> 
> Side-note, but it'd be nice to sort out the inconsistency of PHYS_PFN()
> vs. (end - start) >> PAGE_SHIFT. This is probably not a huge deal though...

I split this out into another patch for easier reviewing.

> 
> >  	}
> >
> >  	return nr_pages;
> > @@ -522,6 +526,7 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
> >  	vms->exec_vm = vms->stack_vm = vms->data_vm = 0;
> >  	vms->unmap_start = FIRST_USER_ADDRESS;
> >  	vms->unmap_end = USER_PGTABLES_CEILING;
> > +	vms->cleared_ptes = false;
> >  }
> >
> >  /*
> > @@ -730,7 +735,6 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  	vma_iter_store(vmi, vma);
> >
> >  	vma_complete(&vp, vmi, vma->vm_mm);
> > -	validate_mm(vma->vm_mm);
> 
> Since we're dropping this here, do we need to re-add this back somehwere
> where we are confident the state will be consistent?

The vma_expand() function is used in two places - one is in the mmap.c
file which can no longer validate the mm until the munmap is complete.
The other is in fs/exec.c which cannot call the validate_mm().  So
to add this call back, I'd have to add a wrapper to vma_expand() to call
the validate_mm() function for debug builds.

Really all this code in fs/exec.c doesn't belong there so we don't need
to do an extra function wrapper just to call validate_mm(). And you have
a patch to do that which is out for review!

> 
> >  	return 0;
> >
> >  nomem:
> > @@ -2612,6 +2616,9 @@ static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
> >  {
> >  	struct mmu_gather tlb;
> >
> > +	if (vms->cleared_ptes)
> > +		return;
> > +
> >  	/*
> >  	 * We can free page tables without write-locking mmap_lock because VMAs
> >  	 * were isolated before we downgraded mmap_lock.
> > @@ -2624,6 +2631,7 @@ static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
> >  	mas_set(mas_detach, 1);
> >  	free_pgtables(&tlb, mas_detach, vms->vma, vms->unmap_start, vms->unmap_end, mm_wr_locked);
> >  	tlb_finish_mmu(&tlb);
> > +	vms->cleared_ptes = true;
> >  }
> >
> >  /*
> > @@ -2936,24 +2944,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  	unsigned long merge_start = addr, merge_end = end;
> >  	bool writable_file_mapping = false;
> >  	pgoff_t vm_pgoff;
> > -	int error;
> > +	int error = -ENOMEM;
> >  	VMA_ITERATOR(vmi, mm, addr);
> > +	unsigned long nr_pages, nr_accounted;
> >
> > -	/* Check against address space limit. */
> > -	if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) {
> > -		unsigned long nr_pages;
> > -
> > -		/*
> > -		 * MAP_FIXED may remove pages of mappings that intersects with
> > -		 * requested mapping. Account for the pages it would unmap.
> > -		 */
> > -		nr_pages = count_vma_pages_range(mm, addr, end);
> > -
> > -		if (!may_expand_vm(mm, vm_flags,
> > -					(len >> PAGE_SHIFT) - nr_pages))
> > -			return -ENOMEM;
> > -	}
> > +	nr_pages = count_vma_pages_range(mm, addr, end, &nr_accounted);
> >
> > +	/* Check against address space limit. */
> > +	/*
> > +	 * MAP_FIXED may remove pages of mappings that intersects with requested
> > +	 * mapping. Account for the pages it would unmap.
> > +	 */
> 
> Utter pedantry, but could these comments be combined? Bit ugly to have one
> after another like this.

Since this was mainly a relocation, I didn't want to change it too much
but since you asked, I'll do it.

> 
> > +	if (!may_expand_vm(mm, vm_flags, (len >> PAGE_SHIFT) - nr_pages))
> > +		return -ENOMEM;
> >
> >  	if (unlikely(!can_modify_mm(mm, addr, end)))
> >  		return -EPERM;
> > @@ -2971,14 +2974,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		if (vms_gather_munmap_vmas(&vms, &mas_detach))
> >  			return -ENOMEM;
> >
> > -		if (vma_iter_clear_gfp(&vmi, addr, end, GFP_KERNEL))
> > -			return -ENOMEM;
> > -
> > -		vms_complete_munmap_vmas(&vms, &mas_detach);
> >  		next = vms.next;
> >  		prev = vms.prev;
> >  		vma = NULL;
> >  	} else {
> > +		/* Minimal setup of vms */
> > +		vms.nr_pages = 0;
> 
> I'm not a huge fan of having vms be uninitialised other than this field and
> then to rely on no further code change accidentally using an uninitialised
> field. This is kind of asking for bugs.
> 
> Can we not find a way to sensibly initialise it somehow?

Yes, I can switch to the same sort of thing as the maple state and
initialize things as empty.

> 
> >  		next = vma_next(&vmi);
> >  		prev = vma_prev(&vmi);
> >  		if (prev)
> > @@ -2990,8 +2991,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  	 */
> >  	if (accountable_mapping(file, vm_flags)) {
> >  		charged = len >> PAGE_SHIFT;
> > +		charged -= nr_accounted;
> >  		if (security_vm_enough_memory_mm(mm, charged))
> > -			return -ENOMEM;
> > +			goto abort_munmap;
> > +		vms.nr_accounted = 0;
> 
> This is kind of expanding the 'vms possibly unitialised apart from selected
> fields' pattern, makes me worry.

I'll fix this with an init of the struct that will always be called.

> 
> >  		vm_flags |= VM_ACCOUNT;
> >  	}
> >
> > @@ -3040,10 +3043,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  	 * not unmapped, but the maps are removed from the list.
> >  	 */
> >  	vma = vm_area_alloc(mm);
> > -	if (!vma) {
> > -		error = -ENOMEM;
> > +	if (!vma)
> >  		goto unacct_error;
> > -	}
> >
> >  	vma_iter_config(&vmi, addr, end);
> >  	vma_set_range(vma, addr, end, pgoff);
> > @@ -3052,6 +3053,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >
> >  	if (file) {
> >  		vma->vm_file = get_file(file);
> > +		/* call_mmap() map PTE, so ensure there are no existing PTEs */
> 
> Typo? Should this be 'call_mmap() maps PTEs, so ensure there are no
> existing PTEs'? I feel like this could be reworded something like:
> 
> 'call_map() may map PTEs, so clear any that may be pending unmap ahead of
> time.'

I had changed this already to 'call_mmap() may map PTE, so ensure there
are no existing PTEs'  That way it's still one line and more descriptive
than what I had.

> 
> > +		if (vms.nr_pages)
> > +			vms_complete_pte_clear(&vms, &mas_detach, true);
> >  		error = call_mmap(file, vma);
> >  		if (error)
> >  			goto unmap_and_free_vma;
> > @@ -3142,6 +3146,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  expanded:
> >  	perf_event_mmap(vma);
> >
> > +	if (vms.nr_pages)
> > +		vms_complete_munmap_vmas(&vms, &mas_detach);
> > +
> 
> Hang on, if we already did this in the if (file) branch above, might we end
> up calling this twice? I didn't see vms.nr_pages get set to zero or
> decremented anywhere (unless I missed it)?

No, we called the new helper vms_complete_pte_clear(), which will avoid
clearing the ptes by the added flag vms->cleared_ptes in the second
call.

Above, I modified vms_complete_pte_clear() to check vms->cleared_ptes
prior to clearing the ptes, so it will only be cleared if it needs
clearing.

I debated moving this nr_pages check within vms_complete_munmap_vmas(),
but that would add an unnecessary check to the munmap() path.  Avoiding
both checks seemed too much code (yet another static inline, or such).
I also wanted to keep the sanity of nr_pages checking to a single
function - as you highlighted it could be a path to insanity.

Considering I'll switch this ti a VMS_INIT(), I think that I could pass
it through and do the logic within the static inline at the expense of
the munmap() having a few extra instructions (but no cache hits, so not
a really big deal).

> 
> >  	vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
> >  	if (vm_flags & VM_LOCKED) {
> >  		if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
> > @@ -3189,6 +3196,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  unacct_error:
> >  	if (charged)
> >  		vm_unacct_memory(charged);
> > +
> > +abort_munmap:
> > +	if (vms.nr_pages)
> > +		abort_munmap_vmas(&mas_detach);
> >  	validate_mm(mm);
> >  	return error;
> >  }
> > --
> > 2.43.0
> >
> 
> In general I like the approach and you've made it very clear how you've
> altered this behaviour.
> 
> However I have a few concerns (as well some trivial comments) above. With
> those cleared up we'll be good to go!