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

[David Hildenbrand]

On 03.03.22 02:47, John Hubbard wrote:
> On 3/2/22 12:38, David Hildenbrand wrote:
> ...
>> 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,
>>
> 
> On this aspect, I think it is easier than trying to discern user
> intentions. Because it is less a question of what the user wants, and
> more a question of how mmap(2) is specified. And the man page clearly
> indicates that the user has no right to expect to see file
> modifications. Here's the excerpt:
> 
> "MAP_PRIVATE
> 	
> Create  a private copy-on-write mapping.  Updates to the mapping are not
> visible to other processes mapping the same file, and are not carried
> through to the underlying file.  It is unspecified whether changes  made
> to the file after the mmap() call are visible in the mapped region.
> "
> 
>> 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.
> 
> Yes, and in fact, I've always thought that the way this was written
> means that it should be treated as a snapshot of the file contents,
> and no longer reliably connected in either direction to the page(s).

Thanks John, that's extremely helpful. I forgot about these MAP_PRIVATE
mmap() details -- they help a lot to clarify which semantics to provide.

So what we could do is:

a) Extend FAULT_FLAG_UNSHARE to also unshare an !anon page in
   a MAP_RPIVATE mapping, replacing it with an (exclusive) anon page.
   R/O PTE permissions are maintained, just like unsharing in the
   context of this series.

b) Similarly trigger FAULT_FLAG_UNSHARE from GUP when trying to take a
   R/O pin (FOLL_PIN) on a R/O-mapped !anon page in a MAP_PRIVATE
   mapping.

c) Make R/O pins consistently use "FOLL_PIN" instead, getting rid of
   FOLL_FORCE|FOLL_WRITE.


Of course, we can't detect MAP_PRIVATE vs. MAP_SHARED in GUP-fast (no
VMA), so we'd always have to fallback in GUP-fast in case we intend to
FOLL_PIN a R/O-mapped !anon page. That would imply that essentially any
R/O pins (FOLL_PIN) would have to fallback to ordinary GUP. BUT, I mean
we require FOLL_FORCE|FOLL_WRITE right now, which is not any different,
so ...

One optimization would be to trigger b) only for FOLL_LONGTERM. For
!FOLL_LONGTERM there are "in theory" absolutely no guarantees which data
will be observed if we modify concurrently to e.g., O_DIRECT IMHO. But
that would require some more thought.

Of course, that's all material for another journey, although it should
be mostly straight forward.

-- 
Thanks,

David / dhildenb