linux-kernel - Re: [PATCH] mm/userfaultfd: Support operation on multiple VMAs

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Date:   Mon, 13 Feb 2023 20:04:45 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     David Hildenbrand <david@...hat.com>, peterx@...hat.com,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        kernel@...labora.com, Paul Gofman <pgofman@...eweavers.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: Support operation on multiple VMAs

Hi David,

Thank you for quick review!

On 2/13/23 4:44 PM, David Hildenbrand wrote:
> On 13.02.23 11:43, Muhammad Usama Anjum wrote:
>> mwriteprotect_range() errors out if [start, end) doesn't fall in one
>> VMA. We are facing a use case where multiple VMAs are present in one
>> range of interest. For example, the following pseudocode reproduces the
>> error which we are trying to fix:
>>
>> - Allocate memory of size 16 pages with PROT_NONE with mmap
>> - Register userfaultfd
>> - Change protection of the first half (1 to 8 pages) of memory to
>>    PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
>> - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
>>    out.
>>
>> This is a simple use case where user may or may not know if the memory
>> area has been divided into multiple VMAs.
>>
>> Reported-by: Paul Gofman <pgofman@...eweavers.com>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
>> ---
>>   mm/userfaultfd.c | 36 +++++++++++++++++++-----------------
>>   1 file changed, 19 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 65ad172add27..46e0a014af68 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -738,9 +738,11 @@ int mwriteprotect_range(struct mm_struct *dst_mm,
>> unsigned long start,
>>               unsigned long len, bool enable_wp,
>>               atomic_t *mmap_changing)
>>   {
>> +    unsigned long end = start + len;
>>       struct vm_area_struct *dst_vma;
>>       unsigned long page_mask;
>>       int err;
>> +    VMA_ITERATOR(vmi, dst_mm, start);
>>         /*
>>        * Sanitize the command parameters:
>> @@ -762,26 +764,26 @@ int mwriteprotect_range(struct mm_struct *dst_mm,
>> unsigned long start,
>>       if (mmap_changing && atomic_read(mmap_changing))
>>           goto out_unlock;
>>   -    err = -ENOENT;
>> -    dst_vma = find_dst_vma(dst_mm, start, len);
>> -
>> -    if (!dst_vma)
>> -        goto out_unlock;
>> -    if (!userfaultfd_wp(dst_vma))
>> -        goto out_unlock;
>> -    if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
>> -        goto out_unlock;
>> +    for_each_vma_range(vmi, dst_vma, end) {
>> +        err = -ENOENT;
>>   -    if (is_vm_hugetlb_page(dst_vma)) {
>> -        err = -EINVAL;
>> -        page_mask = vma_kernel_pagesize(dst_vma) - 1;
>> -        if ((start & page_mask) || (len & page_mask))
>> -            goto out_unlock;
>> -    }
>> +        if (!dst_vma->vm_userfaultfd_ctx.ctx)
>> +            break;
>> +        if (!userfaultfd_wp(dst_vma))
>> +            break;
>> +        if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
>> +            break;
>>   -    uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);
>> +        if (is_vm_hugetlb_page(dst_vma)) {
>> +            err = -EINVAL;
>> +            page_mask = vma_kernel_pagesize(dst_vma) - 1;
>> +            if ((start & page_mask) || (len & page_mask))
>> +                break;
>> +        }
>>   -    err = 0;
>> +        uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);
> 
> I suspect you should be adjusting the range to only cover that specific VMA
> here.
Sorry, you are right. I don't know why it is still working with the
blunder. Will send a v2.

Thanks,
Usama

-- 
BR,
Muhammad Usama Anjum