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proc.5: Document /proc/sys/vm/sysctl_hugetlb_shm_group 

As a deprecated feature, it appears that the RLIMIT_MEMLOCK
can also be used to permit huge page allocation, but let's
not document that for now.

In the Linux 5.12, see fs/hugetlbfs/inode.c.

static int can_do_hugetlb_shm(void)
{
        kgid_t shm_group;
        shm_group = make_kgid(&init_user_ns, sysctl_hugetlb_shm_group);
        return capable(CAP_IPC_LOCK) || in_group_p(shm_group);
}

...

struct file *hugetlb_file_setup(const char *name, size_t size,
                                vm_flags_t acctflag, struct user_struct **user,
                                int creat_flags, int page_size_log)
{
        ...
        if (creat_flags == HUGETLB_SHMFS_INODE && !can_do_hugetlb_shm()) {
                *user = current_user();
                if (user_shm_lock(size, *user)) {
                        task_lock(current);
                        pr_warn_once("%s (%d): Using mlock ulimits for SHM_HUGETLB is deprecated\n",
                                current->comm, current->pid);
                        task_unlock(current);
                } else {
                        *user = NULL;
                        return ERR_PTR(-EPERM);
                }
        }
        ...
}

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2021-05-17 15:18:10 +12:00
parent 3dcdef9437
commit 6cee0ddeb4
1 changed files with 13 additions and 0 deletions
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man5/proc.5
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@ -5603,6 +5603,19 @@ user should run
.BR sync (1)
first.
.TP
.IR /proc/sys/vm/sysctl_hugetlb_shm_group " (since Linux 2.6.7)"
This writable file contains a group ID that is allowed
to allocate memory using huge pages.
If a process has a filesystem group ID or any supplememtary group ID that
matches this group ID,
then it can make huge-page allocations without holding the
.BR CAP_IPC_LOCK
capability; see
.BR memfd_create (2),
.BR mmap (2),
and
.BR shmget (2).
.TP
.IR /proc/sys/vm/legacy_va_layout " (since Linux 2.6.9)"
.\" The following is from Documentation/filesystems/proc.txt
If nonzero, this disables the new 32-bit memory-mapping layout;