Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page

[David Hildenbrand <david@redhat.com>]

On 02.03.22 17:55, Jason Gunthorpe wrote:
> On Thu, Feb 24, 2022 at 01:26:13PM +0100, David Hildenbrand wrote:
>> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
>> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
>> on the page table entry will end up replacing the mapped anonymous page
>> due to COW, resulting in the GUP pin no longer being consistent with the
>> page actually mapped into the page table.
>>
>> The possible ways to deal with this situation are:
>>  (1) Ignore and pin -- what we do right now.
>>  (2) Fail to pin -- which would be rather surprising to callers and
>>      could break user space.
>>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
>>      pins.
> 

Hi Jason,

> How does this mesh with the common FOLL_FORCE|FOLL_WRITE|FOLL_PIN
> pattern used for requesting read access? Can they be converted to
> just FOLL_WRITE|FOLL_PIN after this?

Interesting question, I thought about this in detail yet, let me give it
a try:


IIRC, the sole purpose of FOLL_FORCE in the context of R/O pins is to
enforce the eventual COW -- meaning we COW (via FOLL_WRITE) even if we
don't have the permissions to write (via FOLL_FORCE), to make sure we
most certainly have an exclusive anonymoous page in a MAP_PRIVATE mapping.

Dropping only the FOLL_FORCE would make the FOLL_WRITE request fail if
the mapping is currently !VM_WRITE (but is VM_MAYWRITE), so that
wouldn't work.

I recall that we don't allow pinning the zero page ("special pte",
!vm_normal_page()). So if you have an ordinary MAP_PRIVATE|MAP_ANON
mapping, you will now only need a "FOLL_READ" and have a reliable pin,
even if not previously writing to every page.


It would we different with other MAP_PRIVATE file mappings I remember:

With FOLL_FORCE|FOLL_WRITE|FOLL_PIN we'd force placement of an anonymous
page, resulting in the R/O (long-term ?) pin not observing consecutive
file changes. With a pure FOLL_READ we'd still observe file changes as
we don't trigger a write fault.

BUT, once we actually write to the private mapping via the page table,
the GUP pin would go out of sync with the now-anonymous page mapped into
the page table. However, I'm having a hard time answering what's
actually expected?

It's really hard to tell what the user wants with MAP_PRIVATE file
mappings and stumbles over a !anon page (no modifications so far):

(a) I want a R/O pin to observe file modifications.
(b) I want the R/O pin to *not* observe file modifications but observe
    my (eventual? if any) private modifications,

Of course, if we already wrote to that page and now have an anon page,
it's easy: we are already no longer following file changes.

Maybe FOLL_PIN would already do now what we'd expect from a R/O pin --
(a), maybe not. I'm wondering if FOLL_LONGTERM could give us an
indication whether (a) or (b) applies.

-- 
Thanks,

David / dhildenb