Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

[Peter Xu <peterx@redhat.com>]

On Wed, Apr 07, 2021 at 03:21:55PM +0200, Vlastimil Babka wrote:
> 2) For backports go with the original approach of 17839856fd58 ("gup: document
> and work around "COW can break either way" issue"), thus break COW during the
> GUP. But only for vmplice() so that nothing else gets broken. I think 5.4 stable
> (another LTS) actually backported only 17839856fd58 out of everything else, so
> it should have even the THP case covered, but its userfaultfd() is now probably
> broken...

Since you mentioned this approach - AFAIU userfaultfd was only broken because
with that approach the kernel pretends some read accesses as writes, while
userfaultfd needs that accurate resolution.  Adding something like
FOLL_BREAK_COW [1] upon 17839856fd58 should keep both the vmsplice issue fixed
but also uffd working since that'll keep the read/write operation separate.

Meanwhile, I know Andrea was actively working on a complete solution [2] that's
a few steps further.  E.g., FOLL_BREAK_COW is done with FOLL_UNSHARE [3], speed
up in COW path [4] with similar idea of what we do right now with latest
upstream in 09854ba94c6aad7, allow write-protect with pinned pages (which is
right now forbidden), and something more.  However that's definitely a huge
branch, even discussing upstream (or maybe stopped discussing for quite some
days already?).

Neither of above are within upstream, so I don't really know whether these
information could be anything useful, just raise it up.  If Android could drop
userfaultfd, then I think solution 2) above is indeed the most efficient.  Note
that I think only uffd-wp was affected by 17839856fd58 but not the "missing
mode", so if Android is only using missing mode it still looks fine to only
have 17839856fd58.  It's just that I remembered there's another report besides
uffd-wp on 17839856fd58, but I can't remember the details of the other report.

Thanks,

[1] https://lkml.org/lkml/2020/8/10/439
[2] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/log/?h=mapcount_deshare
[3] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?h=mapcount_deshare&id=7c3a31caa34ac6ac4a4ec0559b1307b5edfc0821
[4] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?h=mapcount_deshare&id=599aa62474f51a470408b28fd4365320a5357aca

-- 
Peter Xu