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List-Subscribe: List-Unsubscribe: On Wed, Jul 16, 2025 at 04:58:04PM +0200, Antonio Quartulli wrote: > I just found this issue in the last linux-next Coverity report and it > caught my attention. > I am not familiar with this code, therefore I am sending this patch > as RFC because I am not 100% sure whether this is a false positive or > not. > However, it seems potentially legit to me: > > In __folio_split(), when looping over folios we dereference > `mapping` before ensuring it is non-NULL. > > Following code in the loop body performs such check, thus > suggesting that `mapping` may be NULL and accessing it > without any check may be dangerous. > > Add NULL check before passing it to shmem_mapping(). > > Cc: Zi Yan > Fixes: 00527733d0dc ("mm/huge_memory: add two new (not yet used) functions for folio_split()") > Addresses-Coverity-ID: 1647614 ("FORWARD_NULL") > Signed-off-by: Antonio Quartulli Thanks for the patch :) but sorry it is in practice not a thing that can happen. See below for analysis. > --- > mm/huge_memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 389620c65a5f..d649026db95a 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3802,7 +3802,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > > /* Some pages can be beyond EOF: drop them from cache */ > if (new_folio->index >= end) { It's kind of not _hugely_ clear but, if anon - which implies no mapping - we set end == -1. At which point this condition cannot evaluate true (index is at page granularity so even MAX_UINT64 would be page shifted and still not equal -1). Under all other circumstances, mapping will be non-NULL. > - if (shmem_mapping(mapping)) > + if (mapping && shmem_mapping(mapping)) > nr_shmem_dropped += folio_nr_pages(new_folio); > else if (folio_test_clear_dirty(new_folio)) > folio_account_cleaned( > -- > 2.49.1 > > Cheers, Lorenzo