Re: [RFC PATCH 1/5] mm: memory: extend finish_fault() to support large folio

[Baolin Wang]

On 2024/4/23 19:03, Ryan Roberts wrote:
> On 22/04/2024 08:02, Baolin Wang wrote:
> > Add large folio mapping establishment support for finish_fault() as a preparation,
> > to support multi-size THP allocation of anonymous shared pages in the following
> > patches.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/memory.c | 25 ++++++++++++++++++-------
> >   1 file changed, 18 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b6fa5146b260..094a76730776 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4766,7 +4766,10 @@ 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;
> > +	int nr_pages, i;
> > +	unsigned long addr;
> >   
> >   	/* Did we COW the page? */
> >   	if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
> > @@ -4797,22 +4800,30 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> >   			return VM_FAULT_OOM;
> >   	}
> >   
> > +	folio = page_folio(page);
> > +	nr_pages = folio_nr_pages(folio);
> > +	addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>
> I'm not sure this is safe. IIUC, finish_fault() is called for any file-backed
> mapping. So you could have a situation where part of a (regular) file is mapped
> in the process, faults and hits in the pagecache. But the folio returned by the
> pagecache is bigger than the portion that the process has mapped. So you now end
> up mapping beyond the VMA limits? In the pagecache case, you also can't assume
> that the folio is naturally aligned in virtual address space.

Good point. Yes, I think you are right, I need consider the VMA limits, 
and I should refer to the calculations of the start pte and end pte in 
do_fault_around().

> >   	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);
> > -
> > -		set_pte_range(vmf, folio, page, 1, vmf->address);
> > -		ret = 0;
> > -	} else {
> > +	if (nr_pages == 1 && vmf_pte_changed(vmf)) {
> >   		update_mmu_tlb(vma, vmf->address, vmf->pte);
> >   		ret = VM_FAULT_NOPAGE;
> > +		goto unlock;
> > +	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>
> I think you have grabbed this from do_anonymous_page()? But I'm not sure it
> works in the same way here as it does there. For the anon case, if userfaultfd
> is armed, alloc_anon_folio() will only ever allocate order-0. So we end up in

IMO, the userfaultfd validation should do in the vma->vm_ops->fault() 
callback, to make sure the nr_pages is always 1 if userfaultfd is armed.

> the vmf_pte_changed() path, which will allow overwriting a uffd entry. But here,
> there is nothing stopping nr_pages being greater than 1 when there could be a
> uffd entry present, and you will fail due to the pte_range_none() check. (see
> pte_marker_handle_uffd_wp()).

So if we do the userfaultfd validation in ->fault() callback, then here 
we can use the same logic as with anonymous case.