Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set_swapbacked()

[Hugh Dickins]

On Tue, 25 Jun 2024, David Hildenbrand wrote:
> On 25.06.24 07:00, Hugh Dickins wrote:
> > Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==
> > false" has extended folio_add_new_anon_rmap() to use on non-exclusive
> > folios, already visible to others in swap cache and on LRU.
> > 
> > That renders its non-atomic __folio_set_swapbacked() unsafe: it risks
> > overwriting concurrent atomic operations on folio->flags, losing bits
> > added or restoring bits cleared.  Since it's only used in this risky
> > way when folio_test_locked and !folio_test_anon, many such races are
> > excluded; but, for example, isolations by folio_test_clear_lru() are
> > vulnerable, and setting or clearing active.
> > 
> > It could just use the atomic folio_set_swapbacked(); but this function
> > does try to avoid atomics where it can, so use a branch instead: just
> > avoid setting swapbacked when it is already set, that is good enough.
> > (Swapbacked is normally stable once set: lazyfree can undo it, but
> > only later, when found anon in a page table.)
> > 
> > This fixes a lot of instability under compaction and swapping loads:
> > assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double
> > frees - though I've not worked out what races could lead to the latter.
> > 
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> >   mm/rmap.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index df1a43295c85..5394c1178bf1 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio,
> > struct vm_area_struct *vma,
> >    VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> >    VM_BUG_ON_VMA(address < vma->vm_start ||
> >   			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
> > -	__folio_set_swapbacked(folio);
> > +
> > +	if (!folio_test_swapbacked(folio))
> > +		__folio_set_swapbacked(folio);
> >    __folio_set_anon(folio, vma, address, exclusive);
> >   
> >    if (likely(!folio_test_large(folio))) {
> 
> LGTM.
> 
> I'll point out that it's sufficient for a PFN walker to do a tryget + trylock
> to cause trouble.

That surprises me.  I thought a racer's tryget was irrelevant (touching
a different field) and its trylock not a problem, since "we" hold the
folio lock throughout.  If my mental model is too naive there, please
explain in more detail: we all need to understand this better.

> 
> Fortunately isolate_movable_page() will check __folio_test_movable() before
> doing the trylock.
> 
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks,
Hugh