linux-kernel - Re: [PATCH v3 4/5] mm: userfaultfd: don't separate addr + len arguments

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Message-ID: <209840DC-22D3-422E-A035-B7ADCB8E531E@vmware.com>
Date:   Tue, 7 Mar 2023 01:29:57 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Xu <peterx@...hat.com>
CC:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Shuah Khan <shuah@...nel.org>,
        James Houghton <jthoughton@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3 4/5] mm: userfaultfd: don't separate addr + len
 arguments



> On Mar 6, 2023, at 5:19 PM, Peter Xu <peterx@...hat.com> wrote:
> 
> !! External Email
> 
> On Mon, Mar 06, 2023 at 02:50:23PM -0800, Axel Rasmussen wrote:
>> We have a lot of functions which take an address + length pair,
>> currently passed as separate arguments. However, in our userspace API we
>> already have struct uffdio_range, which is exactly this pair, and this
>> is what we get from userspace when ioctls are called.
>> 
>> Instead of splitting the struct up into two separate arguments, just
>> plumb the struct through to the functions which use it (once we get to
>> the mfill_atomic_pte level, we're dealing with single (huge)pages, so we
>> don't need both parts).
>> 
>> Relatedly, for waking, just re-use this existing structure instead of
>> defining a new "struct uffdio_wake_range".
>> 
>> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
>> ---
>> fs/userfaultfd.c              | 107 +++++++++++++---------------------
>> include/linux/userfaultfd_k.h |  17 +++---
>> mm/userfaultfd.c              |  92 ++++++++++++++---------------
>> 3 files changed, 96 insertions(+), 120 deletions(-)
>> 
>> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
>> index b8e328123b71..984b63b0fc75 100644
>> --- a/fs/userfaultfd.c
>> +++ b/fs/userfaultfd.c
>> @@ -95,11 +95,6 @@ struct userfaultfd_wait_queue {
>>      bool waken;
>> };
>> 
>> -struct userfaultfd_wake_range {
>> -     unsigned long start;
>> -     unsigned long len;
>> -};
> 
> Would there still be a difference on e.g. 32 bits systems?
> 
> [...]
> 
>> static __always_inline int validate_range(struct mm_struct *mm,
>> -                                       __u64 start, __u64 len)
>> +                                       const struct uffdio_range *range)
>> {
>>      __u64 task_size = mm->task_size;
>> 
>> -     if (start & ~PAGE_MASK)
>> +     if (range->start & ~PAGE_MASK)
>>              return -EINVAL;
>> -     if (len & ~PAGE_MASK)
>> +     if (range->len & ~PAGE_MASK)
>>              return -EINVAL;
>> -     if (!len)
>> +     if (!range->len)
>>              return -EINVAL;
>> -     if (start < mmap_min_addr)
>> +     if (range->start < mmap_min_addr)
>>              return -EINVAL;
>> -     if (start >= task_size)
>> +     if (range->start >= task_size)
>>              return -EINVAL;
>> -     if (len > task_size - start)
>> +     if (range->len > task_size - range->start)
>>              return -EINVAL;
>>      return 0;
>> }
> 
> Personally I don't like a lot on such a change. :( It avoids one parameter
> being passed over but it can add a lot indirections.
> 
> Do you strongly suggest this?  Shall we move on without this so to not
> block the last patch (which I assume is the one you're looking for)?

Just in case you missed, it is __always_inline, so I presume that from a
generated code point-of-view it is the same.

Having said that, small assignments to local start, let and range variables
would make the code easier to read and reduce the change-set.