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

Peter Xu
2021-04-28T15:56:48Z

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.

>
> 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)?

Thanks,

--
Peter Xu