Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages

[David Hildenbrand]

On 14.07.21 18:08, Peter Xu wrote:
> On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu wrote:
> > When a page allocated using the MAP_SHARED flag is swapped out, its pagemap
> > entry is cleared. In many cases, there is no difference between swapped-out
> > shared pages and newly allocated, non-dirty pages in the pagemap interface.
> >
> > This patch addresses the behaviour and modifies pte_to_pagemap_entry() to
> > make use of the XArray associated with the virtual memory area struct
> > passed as an argument. The XArray contains the location of virtual pages
> > in the page cache, swap cache or on disk. If they are on either of the
> > caches, then the original implementation still works. If not, then the
> > missing information will be retrieved from the XArray.
> >
> > Co-developed-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
> > Signed-off-by: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
> > Co-developed-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
> > Signed-off-by: Carl Waldspurger <carl.waldspurger@xxxxxxxxxxx>
> > Co-developed-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> > Signed-off-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> > Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@xxxxxxxxxxx>
> > ---
> >   fs/proc/task_mmu.c | 37 +++++++++++++++++++++++++++++--------
> >   1 file changed, 29 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index eb97468dfe4c..b17c8aedd32e 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1359,12 +1359,25 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end,
> >   	return err;
> >   }
> >   
> > +static void *get_xa_entry_at_vma_addr(struct vm_area_struct *vma,
> > +		unsigned long addr)
> > +{
> > +	struct inode *inode = file_inode(vma->vm_file);
> > +	struct address_space *mapping = inode->i_mapping;
> > +	pgoff_t offset = linear_page_index(vma, addr);
> > +
> > +	return xa_load(&mapping->i_pages, offset);
> > +}
> > +
> >   static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >   		struct vm_area_struct *vma, unsigned long addr, pte_t pte)
> >   {
> >   	u64 frame = 0, flags = 0;
> >   	struct page *page = NULL;
> >   
> > +	if (vma->vm_flags & VM_SOFTDIRTY)
> > +		flags |= PM_SOFT_DIRTY;
> > +
> >   	if (pte_present(pte)) {
> >   		if (pm->show_pfn)
> >   			frame = pte_pfn(pte);
> > @@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >   			flags |= PM_SOFT_DIRTY;
> >   		if (pte_uffd_wp(pte))
> >   			flags |= PM_UFFD_WP;
> > -	} else if (is_swap_pte(pte)) {
> > +	} else if (is_swap_pte(pte) || shmem_file(vma->vm_file)) {
> >   		swp_entry_t entry;
> > -		if (pte_swp_soft_dirty(pte))
> > -			flags |= PM_SOFT_DIRTY;
> > -		if (pte_swp_uffd_wp(pte))
> > -			flags |= PM_UFFD_WP;
> > -		entry = pte_to_swp_entry(pte);
> > +		if (is_swap_pte(pte)) {
> > +			entry = pte_to_swp_entry(pte);
> > +			if (pte_swp_soft_dirty(pte))
> > +				flags |= PM_SOFT_DIRTY;
> > +			if (pte_swp_uffd_wp(pte))
> > +				flags |= PM_UFFD_WP;
> > +		} else {
> > +			void *xa_entry = get_xa_entry_at_vma_addr(vma, addr);
> > +
> > +			if (xa_is_value(xa_entry))
> > +				entry = radix_to_swp_entry(xa_entry);
> > +			else
> > +				goto out;
> > +		}
> >   		if (pm->show_pfn)
> >   			frame = swp_type(entry) |
> >   				(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
> > @@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
> >   		flags |= PM_FILE;
> >   	if (page && page_mapcount(page) == 1)
> >   		flags |= PM_MMAP_EXCLUSIVE;
> > -	if (vma->vm_flags & VM_SOFTDIRTY)
> > -		flags |= PM_SOFT_DIRTY;
>
> IMHO moving this to the entry will only work for the initial iteration, however
> it won't really help anything, as soft-dirty should always be used in pair with
> clear_refs written with value "4" first otherwise all pages will be marked
> soft-dirty then the pagemap data is meaningless.
>
> After the "write 4" op VM_SOFTDIRTY will be cleared and I expect the test case
> to see all zeros again even with the patch.
>
> I think one way to fix this is to do something similar to uffd-wp: we leave a
> marker in pte showing that this is soft-dirtied pte even if swapped out.

How exactly does such a pte look like? Simply pte_none() with another 
bit set?

> However we don't have a mechanism for that yet in current linux, and the
> uffd-wp series is the first one trying to introduce something like that.

Can you give me a pointer? I'm very interested in learning how to 
identify this case.

--
Thanks,

David / dhildenb