Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_pte()

[Axel Rasmussen <axelrasmussen@google.com>]

On Wed, Apr 28, 2021 at 8:56 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Apr 27, 2021 at 05:58:16PM -0700, Hugh Dickins wrote:
> > On Tue, 27 Apr 2021, Axel Rasmussen wrote:
> >
> > > In a previous commit, we added the mcopy_atomic_install_pte() helper.
> > > This helper does the job of setting up PTEs for an existing page, to map
> > > it into a given VMA. It deals with both the anon and shmem cases, as
> > > well as the shared and private cases.
> > >
> > > In other words, shmem_mcopy_atomic_pte() duplicates a case it already
> > > handles. So, expose it, and let shmem_mcopy_atomic_pte() use it
> > > directly, to reduce code duplication.
> > >
> > > This requires that we refactor shmem_mcopy_atomic_pte() a bit:
> > >
> > > Instead of doing accounting (shmem_recalc_inode() et al) part-way
> > > through the PTE setup, do it afterward. This frees up
> > > mcopy_atomic_install_pte() from having to care about this accounting,
> > > and means we don't need to e.g. shmem_uncharge() in the error path.
> > >
> > > A side effect is this switches shmem_mcopy_atomic_pte() to use
> > > lru_cache_add_inactive_or_unevictable() instead of just lru_cache_add().
> > > This wrapper does some extra accounting in an exceptional case, if
> > > appropriate, so it's actually the more correct thing to use.
> > >
> > > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> >
> > Not quite. Two things.
> >
> > One, in this version, delete_from_page_cache(page) has vanished
> > from the particular error path which needs it.
>
> Agreed.  I also spotted that the set_page_dirty() seems to have been overlooked
> when reusing mcopy_atomic_install_pte(), which afaiu should be move into the
> helper.

I think this is covered: we explicitly call SetPageDirty() just before
returning in shmem_mcopy_atomic_pte(). If I remember correctly from a
couple of revisions ago, we consciously put it here instead of in the
helper because it resulted in simpler code (error handling in
particular, I think?), and not all callers of the new helper need it.

>
> >
> > Two, and I think this predates your changes (so needs a separate
> > fix patch first, for backport to stable? a user with bad intentions
> > might be able to trigger the BUG), in pondering the new error paths
> > and that /* don't free the page */ one in particular, isn't it the
> > case that the shmem_inode_acct_block() on entry might succeed the
> > first time, but atomic copy fail so -ENOENT, then something else
> > fill up the tmpfs before the retry comes in, so that retry then
> > fail with -ENOMEM, and hit the BUG_ON(page) in __mcopy_atomic()?
> >
> > (As I understand it, the shmem_inode_unacct_blocks() has to be
> > done before returning, because the caller may be unable to retry.)
> >
> > What the right fix is rather depends on other uses of __mcopy_atomic():
> > if they obviously cannot hit that BUG_ON(page), you may prefer to leave
> > it in, and fix it here where shmem_inode_acct_block() fails. Or you may
> > prefer instead to delete that "else BUG_ON(page);" - looks as if that
> > would end up doing the right thing.  Peter may have a preference.
>
> To me, the BUG_ON(page) wanted to guarantee mfill_atomic_pte() should have
> consumed the page properly when possible.  Removing the BUG_ON() looks good
> already, it will just stop covering the case when e.g. ret==0.
>
> So maybe slightly better to release the page when shmem_inode_acct_block()
> fails (so as to still keep some guard on the page)?

This second issue, I will take some more time to investigate. :)

>
> Thanks,
>
> --
> Peter Xu
>