Re: [PATCH 3/3] mm: migrate: Stabilise page count when migrating transparent hugepages

[Will Deacon]

On Thu, Jun 08, 2017 at 12:52:07PM +0200, Vlastimil Babka wrote:
> On 06/06/2017 07:58 PM, Will Deacon wrote:
> > When migrating a transparent hugepage, migrate_misplaced_transhuge_page
> > guards itself against a concurrent fastgup of the page by checking that
> > the page count is equal to 2 before and after installing the new pmd.
> > 
> > If the page count changes, then the pmd is reverted back to the original
> > entry, however there is a small window where the new (possibly writable)
> > pmd is installed and the underlying page could be written by userspace.
> > Restoring the old pmd could therefore result in loss of data.
> > 
> > This patch fixes the problem by freezing the page count whilst updating
> > the page tables, which protects against a concurrent fastgup without the
> > need to restore the old pmd in the failure case (since the page count can
> > no longer change under our feet).
> > 
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > ---
> >  mm/migrate.c | 15 ++-------------
> >  1 file changed, 2 insertions(+), 13 deletions(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 89a0a1707f4c..8b21f1b1ec6e 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1913,7 +1913,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> >  	int page_lru = page_is_file_cache(page);
> >  	unsigned long mmun_start = address & HPAGE_PMD_MASK;
> >  	unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;
> > -	pmd_t orig_entry;
> >  
> >  	/*
> >  	 * Rate-limit the amount of data that is being migrated to a node.
> > @@ -1956,8 +1955,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> >  	/* Recheck the target PMD */
> >  	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
> >  	ptl = pmd_lock(mm, pmd);
> > -	if (unlikely(!pmd_same(*pmd, entry) || page_count(page) != 2)) {
> > -fail_putback:
> > +	if (unlikely(!pmd_same(*pmd, entry) || !page_ref_freeze(page, 2))) {
> >  		spin_unlock(ptl);
> >  		mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> >  
> > @@ -1979,7 +1977,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> >  		goto out_unlock;
> >  	}
> >  
> > -	orig_entry = *pmd;
> >  	entry = mk_huge_pmd(new_page, vma->vm_page_prot);
> >  	entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> >  
> > @@ -1996,15 +1993,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> 
> There's a comment above this:
> 
>        /*
>          * Clear the old entry under pagetable lock and establish the new PTE.
>          * Any parallel GUP will either observe the old page blocking on the
>          * page lock, block on the page table lock or observe the new page.
>          * The SetPageUptodate on the new page and page_add_new_anon_rmap
>          * guarantee the copy is visible before the pagetable update.
>          */
> 
> Is it still correct? Didn't the freezing prevent some of the cases above?

I don't think the comment needs to change, the freezing is just doing
correctly what the code tried to do before. Granted, the blocking might come
about because of the count momentarily being set to 0 (and
page_cache_add_speculative bailing), but that's just fastGUP implementation
details, I think.

> 
> >  	set_pmd_at(mm, mmun_start, pmd, entry);
> >  	update_mmu_cache_pmd(vma, address, &entry);
> >  
> > -	if (page_count(page) != 2) {
> 
> BTW, how did the old code recognize that page count would increase and then
> decrease back?

I'm not sure that case matters because the inc/dec would happen before the
new PMD is put in place (otherwise it wouldn't be reachable via the
fastGUP).

Will