linux-kernel - Re: [PATCH v5 06/10] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install

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Date:   Wed, 28 Apr 2021 11:56:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Shaohua Li <shli@...com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Wang Qing <wangqing@...o.com>, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
        Brian Geffon <bgeffon@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v5 06/10] userfaultfd/shmem: modify
 shmem_mcopy_atomic_pte to use install_pte()

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@...gle.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