On 25.02.25 16:54, Vlastimil Babka wrote:
On 2/18/25 18:28, Lorenzo Stoakes wrote:
On Tue, Feb 18, 2025 at 06:25:35PM +0100, David Hildenbrand wrote:
It fails because it tries to 'touch' the memory, but 'touching' guard
region memory causes a segfault. This kind of breaks the idea of
mlock()'ing guard regions.
I think adding workarounds to make this possible in any way is not really
worth it (and would probably be pretty gross).
We already document that 'mlock()ing lightweight guard regions will fail'
as per man page so this is all in line with that.
Right, and I claim that supporting VM_LOCKONFAULT might likely be as easy as
allowing install/remove of guard regions when that flag is set.
We already allow this flag! VM_LOCKED and VM_HUGETLB are the only flags we
disallow.
See mlock2();
SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)
{
vm_flags_t vm_flags = VM_LOCKED;
if (flags & ~MLOCK_ONFAULT)
return -EINVAL;
if (flags & MLOCK_ONFAULT)
vm_flags |= VM_LOCKONFAULT;
return do_mlock(start, len, vm_flags);
}
VM_LOCKONFAULT always as VM_LOCKED set as well.
OK cool, that makes sense.
As with much kernel stuff, I knew this in the past. Then I forgot. Then I knew
again, then... :P if only somebody would write it down in a book...
Yeah then that makes sense to check explicitly for (VM_LOCKED | VM_LOCKONFAULT)
in any MADV_GUARD_INSTALL_LOCKED variant as obviously this would be passively
excluded right now.
Sorry for the late reply. So AFAIU from your conversations, guards can't be
compatible with VM_LOCKED, which means e.g. any attempts of glibc to use
guards for stacks will soon discover that mlockall() users exist and are
broken by this, and the attempts will fail? That's a bummer.
As for compatibility with VM_LOCKONFAULT, do we need a new
MADV_GUARD_INSTALL_LOCKED or can we say MADV_GUARD_INSTALL is new enough
that it can be just retrofitted (like you retrofit file backed mappings)?
AFAIU the only risk would be breaking somebody that already relies on a
failure for VM_LOCKONFAULT, and it's unlikely there's such a somebody now.
Exactly my thinking I didn't have time to phrase during that
conversation. IMHO, we were careful with MADV_DONTNEED because it was
... around for a little bit longer :)
--
Cheers,
David / dhildenb
