Re: [PATCH v3 03/26] mm: introduce AS_NO_DIRECT_MAP

"Brendan Jackman" <[email protected]> writes:

> Right, attempting this again now I've fixed my mail setup.
>
> I've no idea how that other "butt-dial" mail got sent, I was not able to
> sent mail _deliberately_ during that period but I guess somewhere in my
> setup I still had a copy of the password somewhere!
>
> On Fri Jul 31, 2026 at 8:28 PM BST, Yosry Ahmed wrote:
>>> > [..]
>
>>> >>  		return -EFAULT;
>>> >>
>>> >>  	if (write) {
>>> >> @@ -2731,7 +2730,7 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>>> >>   * This call assumes the caller has pinned the folio, that the lowest page table
>>> >>   * level still points to this folio, and that interrupts have been disabled.
>>> >>   *
>>> >> - * GUP-fast must reject all secretmem folios.
>>> >> + * GUP-fast must reject all folios without direct map entries (such as secretmem).
>>> >>   *
>>> >>   * Writing to pinned file-backed dirty tracked folios is inherently problematic
>>> >>   * (see comment describing the writable_file_mapping_allowed() function). We
>>> >> @@ -2769,7 +2768,7 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>>> >>  	if (WARN_ON_ONCE(folio_test_slab(folio)))
>>> >>  		return false;
>>> >>
>>> >> -	/* hugetlb neither requires dirty-tracking nor can be secretmem. */
>>> >> +	/* hugetlb neither requires dirty-tracking nor can be without direct map. */
>>
>> Is this necessarily true? I know there were discussions/proposals about
>> using some of the hugetlb infrastructure for guest_memfd. I am not sure
>> if those folios would remain hugetlb folios though.
>>
>> Adding Ackerley here.
>
> I think Google might have a bunch of patches for this, afraid I know
> very little about it though.
>

We do have guest_memfd HugeTLB support (I'm gonna work on upstreaming!!)

When guest_memfd gets direct map removal support then at some point
guest_memfd HugeTLB is going to have to respect that. For now, I'd say
we should write this patch series assuming guest_memfd doesn't have
direct map removal support or HugeTLB pages.

>>> >>  	if (folio_test_hugetlb(folio))
>>> >>  		return true;
>>> >>
>>>
>>> [...snip...]
>>>