The patch titled Subject: mm: hugetlb pages should not be reserved by shmat() if SHM_NORESERVE has been added to the -mm mm-hotfixes-unstable branch. Its filename is hugetlb-pages-should-not-be-reserved-by-shmat-if-shm_noreserve.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: Prakash Sangappa prakash.sangappa@oracle.com Subject: mm: hugetlb pages should not be reserved by shmat() if SHM_NORESERVE Date: Tue, 23 Jan 2024 12:04:42 -0800

For shared memory of type SHM_HUGETLB, hugetlb pages are reserved in shmget() call. If SHM_NORESERVE flags is specified then the hugetlb pages are not reserved. However when the shared memory is attached with the shmat() call the hugetlb pages are getting reserved incorrectly for SHM_HUGETLB shared memory created with SHM_NORESERVE which is a bug.

------------------------------- Following test shows the issue.

$cat shmhtb.c

int main() { int shmflags = 0660 | IPC_CREAT | SHM_HUGETLB | SHM_NORESERVE; int shmid;

shmid = shmget(SKEY, SHMSZ, shmflags); if (shmid < 0) { printf("shmat: shmget() failed, %d\n", errno); return 1; } printf("After shmget()\n"); system("cat /proc/meminfo | grep -i hugepages_");

shmat(shmid, NULL, 0); printf("\nAfter shmat()\n"); system("cat /proc/meminfo | grep -i hugepages_");

shmctl(shmid, IPC_RMID, NULL); return 0; }

#sysctl -w vm.nr_hugepages=20 #./shmhtb

After shmget() HugePages_Total: 20 HugePages_Free: 20 HugePages_Rsvd: 0 HugePages_Surp: 0

After shmat() HugePages_Total: 20 HugePages_Free: 20 HugePages_Rsvd: 5 <-- HugePages_Surp: 0 --------------------------------

Fix is to ensure that hugetlb pages are not reserved for SHM_HUGETLB shared memory in the shmat() call.

Link: https://lkml.kernel.org/r/1706040282-12388-1-git-send-email-prakash.sangappa... Signed-off-by: Prakash Sangappa prakash.sangappa@oracle.com Acked-by: Muchun Song muchun.song@linux.dev Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/hugetlbfs/inode.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)

--- a/fs/hugetlbfs/inode.c~hugetlb-pages-should-not-be-reserved-by-shmat-if-shm_noreserve +++ a/fs/hugetlbfs/inode.c @@ -100,6 +100,7 @@ static int hugetlbfs_file_mmap(struct fi loff_t len, vma_len; int ret; struct hstate *h = hstate_file(file); + vm_flags_t vm_flags;

/* * vma address alignment (but not the pgoff alignment) has @@ -141,10 +142,20 @@ static int hugetlbfs_file_mmap(struct fi file_accessed(file);

ret = -ENOMEM; + + vm_flags = vma->vm_flags; + /* + * for SHM_HUGETLB, the pages are reserved in the shmget() call so skip + * reserving here. Note: only for SHM hugetlbfs file, the inode + * flag S_PRIVATE is set. + */ + if (inode->i_flags & S_PRIVATE) + vm_flags |= VM_NORESERVE; + if (!hugetlb_reserve_pages(inode, vma->vm_pgoff >> huge_page_order(h), len >> huge_page_shift(h), vma, - vma->vm_flags)) + vm_flags)) goto out;

ret = 0; _

Patches currently in -mm which might be from prakash.sangappa@oracle.com are

hugetlb-pages-should-not-be-reserved-by-shmat-if-shm_noreserve.patch