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

[Hugh Dickins <hughd@google.com>]

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.

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.

(Or, we could consider doing the shmem_inode_acct_block() only after
the page has been copied in: its current placing reflects how shmem.c
does it elsewhere, and there's reason for that, but it doesn't always
work out right. Don't be surprised if I change the ordering in future,
but it's probably best not to mess with that ordering now.)

Sorry, if this is a pre-existing issue, then we are taking advantage
of you, in asking you to fix it: but I hope that while you're in there,
it will make sense to do so.

Thanks,
Hugh

> ---
>  include/linux/userfaultfd_k.h |  5 ++++
>  mm/shmem.c                    | 48 +++++------------------------------
>  mm/userfaultfd.c              | 17 +++++--------
>  3 files changed, 18 insertions(+), 52 deletions(-)