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This is done by first pinning all of the target folios > anyway, collecting all of the longterm-unpinnable target folios into a > list, dropping the pins that were just taken and finally handing the > list off to migrate_pages() for the actual migration. > > It is critically important that no unexpected references are held on the > folios being migrated, otherwise the migration will fail and > pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is > relatively easy to observe migration failures when running pKVM (which > uses pin_user_pages() on crosvm's virtual address space to resolve > stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and > this results in the VM terminating prematurely. > > In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its > mapping of guest memory prior to the pinning. Subsequently, when > pin_user_pages() walks the page-table, the relevant 'pte' is not > present and so the faulting logic allocates a new folio, mlocks it > with mlock_folio() and maps it in the page-table. > > Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() > batch by pagevec"), mlock/munlock operations on a folio (formerly page), > are deferred. For example, mlock_folio() takes an additional reference > on the target folio before placing it into a per-cpu 'folio_batch' for > later processing by mlock_folio_batch(), which drops the refcount once > the operation is complete. Processing of the batches is coupled with > the LRU batch logic and can be forcefully drained with > lru_add_drain_all() but as long as a folio remains unprocessed on the > batch, its refcount will be elevated. > > This deferred batching therefore interacts poorly with the pKVM pinning I would go even a little broader (more general), and claim that this deferred batching interacts poorly with gup FOLL_LONGTERM when trying to pin folios in CMA or ZONE_MOVABLE, in fact. More on this below. > scenario as we can find ourselves in a situation where the migration > code fails to migrate a folio due to the elevated refcount from the > pending mlock operation. > > Extend the existing LRU draining logic in > collect_longterm_unpinnable_folios() so that unpinnable mlocked folios > on the LRU also trigger a drain. > > Cc: Hugh Dickins > Cc: Keir Fraser > Cc: Jason Gunthorpe > Cc: David Hildenbrand > Cc: John Hubbard > Cc: Frederick Mayle > Cc: Andrew Morton > Cc: Peter Xu > Fixes: 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() batch by pagevec") > Signed-off-by: Will Deacon > --- > > This has been quite unpleasant to debug and, as I'm not intimately I'll bet! It's not even pleasant to *read* about it! haha. Sorry you had to suffer through this. > familiar with the mm internals, I've tried to include all the relevant > details in the commit message in case there's a preferred alternative > way of solving the problem or there's a flaw in my logic. > > mm/gup.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/gup.c b/mm/gup.c > index adffe663594d..656835890f05 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_unpinnable_folios( > continue; > } > > - if (!folio_test_lru(folio) && drain_allow) { > + if (drain_allow && > + (!folio_test_lru(folio) || folio_test_mlocked(folio))) { That should work, yes. Alternatively, after thinking about this a bit today, it seems to me that the mlock batching is a little too bold, given the presence of gup/pup. And so I'm tempted to fix the problem closer to the root cause, like this (below). But maybe this is actually *less* wise than what you have proposed... I'd like to hear other mm folks' opinion on this approach: diff --git a/mm/mlock.c b/mm/mlock.c index a1d93ad33c6d..edecdd32996e 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -278,7 +278,15 @@ void mlock_new_folio(struct folio *folio) folio_get(folio); if (!folio_batch_add(fbatch, mlock_new(folio)) || - folio_test_large(folio) || lru_cache_disabled()) + folio_test_large(folio) || lru_cache_disabled() || + /* + * If this is being called as part of a gup FOLL_LONGTERM operation in + * CMA/MOVABLE zones with MLOCK_ONFAULT active, then the newly faulted + * in folio will need to immediately migrate to a pinnable zone. + * Allowing the mlock operation to batch would break the ability to + * migrate the folio. Instead, force immediate processing. + */ + (current->flags & PF_MEMALLOC_PIN)) mlock_folio_batch(fbatch); local_unlock(&mlock_fbatch.lock); } > lru_add_drain_all(); > drain_allow = false; > } thanks, -- John Hubbard