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List-Subscribe: List-Unsubscribe: On 6/11/25 6:13 AM, David Hildenbrand wrote: > After commit 1aaf8c122918 ("mm: gup: fix infinite loop within > __get_longterm_locked") we are able to longterm pin folios that are not > supposed to get longterm pinned, simply because they temporarily have > the LRU flag cleared (esp. temporarily isolated). > > For example, two __get_longterm_locked() callers can race, or > __get_longterm_locked() can race with anything else that temporarily > isolates folios. > > The introducing commit mentions the use case of a driver that uses > vm_ops->fault to insert pages allocated through cma_alloc() into the > page tables, assuming they can later get longterm pinned. These pages/ > folios would never have the LRU flag set and consequently cannot get > isolated. There is no known in-tree user making use of that so far, > fortunately. > > To handle that in the future -- and avoid retrying forever to > isolate/migrate them -- we will need a different mechanism for the CMA > area *owner* to indicate that it actually already allocated the page and > is fine with longterm pinning it. The LRU flag is not suitable for that. > > Probably we can lookup the relevant CMA area and query the bitmap; we > only have have to care about some races, probably. If already allocated, > we could just allow longterm pinning) > > Anyhow, let's fix the "must not be longterm pinned" problem first by > reverting the original commit. Really great commit description, I appreciate the time and effort spent summarizing what happened here. > > Fixes: 1aaf8c122918 ("mm: gup: fix infinite loop within __get_longterm_locked") > Closes: https://lore.kernel.org/all/20250522092755.GA3277597@tiffany/ > Reported-by: Hyesoo Yu > Cc: > Cc: Andrew Morton > Cc: Jason Gunthorpe > Cc: Peter Xu > Cc: Zhaoyang Huang > Cc: Aijun Sun > Cc: Alistair Popple > Cc: John Hubbard > Signed-off-by: David Hildenbrand > --- > mm/gup.c | 14 ++++++++++---- > 1 file changed, 10 insertions(+), 4 deletions(-) > I've verified that this is an exact revert of 1aaf8c122918, yes. Reviewed-by: John Hubbard thanks, -- John Hubbard > diff --git a/mm/gup.c b/mm/gup.c > index e065a49842a87..3c39cbbeebef1 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -2303,13 +2303,13 @@ static void pofs_unpin(struct pages_or_folios *pofs) > /* > * Returns the number of collected folios. Return value is always >= 0. > */ > -static void collect_longterm_unpinnable_folios( > +static unsigned long collect_longterm_unpinnable_folios( > struct list_head *movable_folio_list, > struct pages_or_folios *pofs) > { > + unsigned long i, collected = 0; > struct folio *prev_folio = NULL; > bool drain_allow = true; > - unsigned long i; > > for (i = 0; i < pofs->nr_entries; i++) { > struct folio *folio = pofs_get_folio(pofs, i); > @@ -2321,6 +2321,8 @@ static void collect_longterm_unpinnable_folios( > if (folio_is_longterm_pinnable(folio)) > continue; > > + collected++; > + > if (folio_is_device_coherent(folio)) > continue; > > @@ -2342,6 +2344,8 @@ static void collect_longterm_unpinnable_folios( > NR_ISOLATED_ANON + folio_is_file_lru(folio), > folio_nr_pages(folio)); > } > + > + return collected; > } > > /* > @@ -2418,9 +2422,11 @@ static long > check_and_migrate_movable_pages_or_folios(struct pages_or_folios *pofs) > { > LIST_HEAD(movable_folio_list); > + unsigned long collected; > > - collect_longterm_unpinnable_folios(&movable_folio_list, pofs); > - if (list_empty(&movable_folio_list)) > + collected = collect_longterm_unpinnable_folios(&movable_folio_list, > + pofs); > + if (!collected) > return 0; > > return migrate_longterm_unpinnable_folios(&movable_folio_list, pofs);