From: Rik van Riel <riel@xxxxxxxxxxx> commit 8ebe0a5eaaeb099de03d09ad20f54ed962e2261e upstream. A common use case for hugetlbfs is for the application to create memory pools backed by huge pages, which then get handed over to some malloc library (eg. jemalloc) for further management. That malloc library may be doing MADV_DONTNEED calls on memory that is no longer needed, expecting those calls to happen on PAGE_SIZE boundaries. However, currently the MADV_DONTNEED code rounds up any such requests to HPAGE_PMD_SIZE boundaries. This leads to undesired outcomes when jemalloc expects a 4kB MADV_DONTNEED, but 2MB of memory get zeroed out, instead. Use of pre-built shared libraries means that user code does not always know the page size of every memory arena in use. Avoid unexpected data loss with MADV_DONTNEED by rounding up only to PAGE_SIZE (in do_madvise), and rounding down to huge page granularity. That way programs will only get as much memory zeroed out as they requested. Link: https://lkml.kernel.org/r/20221021192805.366ad573@xxxxxxxxxxxxxxxxxxxx Fixes: 90e7e7f5ef3f ("mm: enable MADV_DONTNEED for hugetlb mappings") Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/madvise.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) --- a/mm/madvise.c +++ b/mm/madvise.c @@ -811,7 +811,14 @@ static bool madvise_dontneed_free_valid_ if (start & ~huge_page_mask(hstate_vma(vma))) return false; - *end = ALIGN(*end, huge_page_size(hstate_vma(vma))); + /* + * Madvise callers expect the length to be rounded up to PAGE_SIZE + * boundaries, and may be unaware that this VMA uses huge pages. + * Avoid unexpected data loss by rounding down the number of + * huge pages freed. + */ + *end = ALIGN_DOWN(*end, huge_page_size(hstate_vma(vma))); + return true; } @@ -826,6 +833,9 @@ static long madvise_dontneed_free(struct if (!madvise_dontneed_free_valid_vma(vma, start, &end, behavior)) return -EINVAL; + if (start == end) + return 0; + if (!userfaultfd_remove(vma, start, end)) { *prev = NULL; /* mmap_lock has been dropped, prev is stale */
