The patch titled Subject: mm,madvise,hugetlb: fix unexpected data loss with MADV_DONTNEED on hugetlbfs has been added to the -mm mm-hotfixes-unstable branch. Its filename is mmmadvisehugetlb-fix-unexpected-data-loss-with-madv_dontneed-on-hugetlbfs.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Rik van Riel riel@surriel.com Subject: mm,madvise,hugetlb: fix unexpected data loss with MADV_DONTNEED on hugetlbfs Date: Fri, 21 Oct 2022 19:28:05 -0400

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@imladris.surriel.com Fixes: 90e7e7f5ef3f ("mm: enable MADV_DONTNEED for hugetlb mappings") Reviewed-by: Mike Kravetz mike.kravetz@oracle.com Cc: David Hildenbrand david@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/madvise.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)

--- a/mm/madvise.c~mmmadvisehugetlb-fix-unexpected-data-loss-with-madv_dontneed-on-hugetlbfs +++ a/mm/madvise.c @@ -813,7 +813,14 @@ 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; }

@@ -828,6 +835,9 @@ 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 */

_

Patches currently in -mm which might be from riel@surriel.com are

mmmadvisehugetlb-fix-unexpected-data-loss-with-madv_dontneed-on-hugetlbfs.patch