Re: [PATCH v2 00/24] userfaultfd-wp: Support shmem and hugetlbfs

[Hugh Dickins <hughd@google.com>]

On Wed, 12 May 2021, Peter Xu wrote:
> On Tue, Apr 27, 2021 at 12:12:53PM -0400, Peter Xu wrote:
> > This is v2 of uffd-wp shmem & hugetlbfs support, which completes uffd-wp as a
> > kernel full feature, as it only supports anonymous before this series.
> 
> Ping..
> 
> Thinking about a repost, as this series shouldn't be able to apply after we've
> got more relevant patches into -mm.  E.g., the full minor fault, and also some
> small stuff like pagemap, as we need one more patch to support shmem/hugetlbfs
> too.
> 
> Hugh, haven't received any further comment from you on shmem side (or on the
> general idea).  It would be great to still have some of your input.
> 
> Let me know if you prefer to read a fresh new version otherwise.

I am very sorry to let you down, Peter, repeatedly; but it is now very
clear that I shall *never* have time to review your patchset - I am too
slow, have too much else to attend to, and take too long each time to
sink myself deep enough into userfaultfd.

I realize that you're being considerate, and expecting no more than
a few comments from me, rather than asking for formal review; but it's
still too much for me to get into.

The only reason I was involved at all, was when you were wondering how
to handle the pagetable entries for shmem.  I suggested one encoding,
Andrea suggested slightly differently: Andrea's was more elegant (no
"swap type" required), and it looked like you went with his - good.

I wonder whether you noticed
https://lore.kernel.org/linux-mm/20210407084238.20443-2-apopple@nvidia.com/
which might interfere.  I've had no more time to look at that than yours,
so no opinion on it (and I don't know what happened to it after that).

Please keep uppermost in mind when modifying mm/shmem.c for userfaultfd,
the difference between shared and private; and be on guard against the
ways in which CONFIG_USERFAULTFD=y might open a door to abuse.

Thanks,
Hugh