linux-kernel - Re: [PATCH 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}

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Date:   Mon, 12 Apr 2021 19:57:52 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Colascione <dancol@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        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 2/9] userfaultfd/shmem: combine
 shmem_{mcopy_atomic,mfill_zeropage}_pte

On Thu, Apr 08, 2021 at 04:43:20PM -0700, Axel Rasmussen wrote:
> Previously, we did a dance where we had one calling path in
> userfaultfd.c (mfill_atomic_pte), but then we split it into two in
> shmem_fs.h (shmem_{mcopy_atomic,mfill_zeropage}_pte), and then rejoined
> into a single shared function in shmem.c (shmem_mfill_atomic_pte).
> 
> This is all a bit overly complex. Just call the single combined shmem
> function directly, allowing us to clean up various branches,
> boilerplate, etc.
> 
> While we're touching this function, two other small cleanup changes:
> - offset is equivalent to pgoff, so we can get rid of offset entirely.
> - Split two VM_BUG_ON cases into two statements. This means the line
>   number reported when the BUG is hit specifies exactly which condition
>   was true.

(For my own preference, I'll avoid touching the latter one)

> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> ---
>  include/linux/shmem_fs.h | 15 +++++-------
>  mm/shmem.c               | 52 +++++++++++++---------------------------
>  mm/userfaultfd.c         | 10 +++-----
>  3 files changed, 25 insertions(+), 52 deletions(-)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index d82b6f396588..919e36671fe6 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -122,21 +122,18 @@ static inline bool shmem_file(struct file *file)
>  extern bool shmem_charge(struct inode *inode, long pages);
>  extern void shmem_uncharge(struct inode *inode, long pages);
>  
> +#ifdef CONFIG_USERFAULTFD
>  #ifdef CONFIG_SHMEM
>  extern int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
>  				  struct vm_area_struct *dst_vma,
>  				  unsigned long dst_addr,
>  				  unsigned long src_addr,

Not a problem of your patch, but it's just that we passed in odd src_addr
values into mfill_atomic_pte() for zeropage case because we loop on src_addr in
__mcopy_atomic()...  Then it'll further passed into shmem_mcopy_atomic_pte()
now after this patch (as shmem_mfill_zeropage_pte() probably only did one thing
good which is to clear src_addr).  Not a big deal, though.

All the rest looks sane to me.

Reviewed-by: Peter Xu <peterx@...hat.com>

I'll wait to look at the selftests since in all cases they should be prone to
rebase (either based on the v2 cleanup I posted, or you'd need to post without
err() - then I can rebase again), so I figured maybe I just read the new
version.

Thanks,

-- 
Peter Xu