Re: [PATCH 2/8] mm: memory: extend finish_fault() to support large folio

[Baolin Wang]

On 2024/5/7 18:37, Ryan Roberts wrote:
> On 06/05/2024 09:46, Baolin Wang wrote:
> > Add large folio mapping establishment support for finish_fault() as a preparation,
> > to support multi-size THP allocation of anonymous shmem pages in the following
> > patches.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/memory.c | 43 +++++++++++++++++++++++++++++++++----------
> >   1 file changed, 33 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index eea6e4984eae..936377220b77 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4747,9 +4747,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >   {
> >   	struct vm_area_struct *vma = vmf->vma;
> >   	struct page *page;
> > +	struct folio *folio;
> >   	vm_fault_t ret;
> >   	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
> >   		      !(vma->vm_flags & VM_SHARED);
> > +	int type, nr_pages, i;
> > +	unsigned long addr = vmf->address;
> >   
> >   	/* Did we COW the page? */
> >   	if (is_cow)
> > @@ -4780,24 +4783,44 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >   			return VM_FAULT_OOM;
> >   	}
> >   
> > +	folio = page_folio(page);
> > +	nr_pages = folio_nr_pages(folio);
> > +
> > +	if (unlikely(userfaultfd_armed(vma))) {
> > +		nr_pages = 1;
> > +	} else if (nr_pages > 1) {
> > +		unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
> > +		unsigned long end = start + nr_pages * PAGE_SIZE;
> > +
> > +		/* In case the folio size in page cache beyond the VMA limits. */
> > +		addr = max(start, vma->vm_start);
> > +		nr_pages = (min(end, vma->vm_end) - addr) >> PAGE_SHIFT;
> > +
> > +		page = folio_page(folio, (addr - start) >> PAGE_SHIFT);
>
> I still don't really follow the logic in this else if block. Isn't it possible
> that finish_fault() gets called with a page from a folio that isn't aligned with
> vmf->address?
>
> For example, let's say we have a file who's size is 64K and which is cached in a
> single large folio in the page cache. But the file is mapped into a process at
> VA 16K to 80K. Let's say we fault on the first page (VA=16K). You will calculate

For shmem, this doesn't happen because the VA is aligned with the 
hugepage size in the shmem_get_unmapped_area() function. See patch 7.

> start=0 and end=64K I think?

Yes. Unfortunately, some file systems that support large mappings do not 
perform alignment for multi-size THP (non-PMD sized, for example: 64K). 
I think this requires modification to 
__get_unmapped_area--->thp_get_unmapped_area_vmflags() or 
file->f_op->get_unmapped_area() to align VA for multi-size THP in future.

So before adding that VA alignment changes, only allow building the 
large folio mapping for anonymous shmem:

diff --git a/mm/memory.c b/mm/memory.c
index 936377220b77..9e4d51826d23 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4786,7 +4786,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
        folio = page_folio(page);
        nr_pages = folio_nr_pages(folio);

-       if (unlikely(userfaultfd_armed(vma))) {
+       if (unlikely(userfaultfd_armed(vma)) || !vma_is_anon_shmem(vma)) {
                nr_pages = 1;
        } else if (nr_pages > 1) {
                unsigned long start = ALIGN_DOWN(vmf->address, nr_pages 
* PAGE_SIZE);

> Additionally, I think this path will end up mapping the entire folio (as long as
> it fits in the VMA). But this bypasses the fault-around configuration. As I
> think I mentioned against the RFC, this will inflate the RSS of the process and
> can cause behavioural changes as a result. I believe the current advice is to
> disable fault-around to prevent this kind of bloat when needed.

With above change, I do not think this is a problem? since users already 
want to use mTHP for anonymous shmem.

> It might be that you need a special variant of finish_fault() for shmem?
>
>
> > +	}
> >   	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> > -				      vmf->address, &vmf->ptl);
> > +				       addr, &vmf->ptl);
> >   	if (!vmf->pte)
> >   		return VM_FAULT_NOPAGE;
> >   
> >   	/* Re-check under ptl */
> > -	if (likely(!vmf_pte_changed(vmf))) {
> > -		struct folio *folio = page_folio(page);
> > -		int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
> > -
> > -		set_pte_range(vmf, folio, page, 1, vmf->address);
> > -		add_mm_counter(vma->vm_mm, type, 1);
> > -		ret = 0;
> > -	} else {
> > -		update_mmu_tlb(vma, vmf->address, vmf->pte);
> > +	if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
> > +		update_mmu_tlb(vma, addr, vmf->pte);
> > +		ret = VM_FAULT_NOPAGE;
> > +		goto unlock;
> > +	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> > +		for (i = 0; i < nr_pages; i++)
> > +			update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
> >   		ret = VM_FAULT_NOPAGE;
> > +		goto unlock;
> >   	}
> >   
> > +	set_pte_range(vmf, folio, page, nr_pages, addr);
> > +	type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
> > +	add_mm_counter(vma->vm_mm, type, nr_pages);
> > +	ret = 0;
> > +
> > +unlock:
> >   	pte_unmap_unlock(vmf->pte, vmf->ptl);
> >   	return ret;
> >   }