From: Zach O'Keefe <zokeefe@xxxxxxxxxx> commit ae63c898f4004bbc7d212f4adcb3bb14852c30d6 upstream. During collapse, in a few places we check to see if a given small page has any unaccounted references. If the refcount on the page doesn't match our expectations, it must be there is an unknown user concurrently interested in the page, and so it's not safe to move the contents elsewhere. However, the unaccounted pins are likely an ephemeral state. In this situation, MADV_COLLAPSE returns -EINVAL when it should return -EAGAIN. This could cause userspace to conclude that the syscall failed, when it in fact could succeed by retrying. Link: https://lkml.kernel.org/r/20230125015738.912924-1-zokeefe@xxxxxxxxxx Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/khugepaged.c | 1 + 1 file changed, 1 insertion(+) --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2608,6 +2608,7 @@ static int madvise_collapse_errno(enum s case SCAN_CGROUP_CHARGE_FAIL: return -EBUSY; /* Resource temporary unavailable - trying again might succeed */ + case SCAN_PAGE_COUNT: case SCAN_PAGE_LOCK: case SCAN_PAGE_LRU: case SCAN_DEL_PAGE_LRU:
