Re: BUG: unable to handle kernel paging request in fuse_copy_do

[David Hildenbrand <david@redhat.com>]

On 22.03.24 22:33, David Hildenbrand wrote:
> On 22.03.24 22:18, David Hildenbrand wrote:
>> On 22.03.24 22:13, Miklos Szeredi wrote:
>>> On Fri, 22 Mar 2024 at 22:08, David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 22.03.24 20:46, Miklos Szeredi wrote:
>>>>> On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@redhat.com> wrote:
>>>>>
>>>>>> But at least the vmsplice() just seems to work. Which is weird, because
>>>>>> GUP-fast should not apply (page not faulted in?)
>>>>>
>>>>> But it is faulted in, and that indeed seems to be the root cause.
>>>>
>>>> secretmem mmap() won't populate the page tables. So it's not faulted in yet.
>>>>
>>>> When we GUP via vmsplice, GUP-fast should not find it in the page tables
>>>> and fallback to slow GUP.
>>>>
>>>> There, we seem to pass check_vma_flags(), trigger faultin_page() to
>>>> fault it in, and then find it via follow_page_mask().
>>>>
>>>> ... and I wonder how we manage to skip check_vma_flags(), or otherwise
>>>> managed to GUP it.
>>>>
>>>> vmsplice() should, in theory, never succeed here.
>>>>
>>>> Weird :/
>>>>
>>>>> Improved repro:
>>>>>
>>>>> #define _GNU_SOURCE
>>>>>
>>>>> #include <fcntl.h>
>>>>> #include <unistd.h>
>>>>> #include <stdio.h>
>>>>> #include <errno.h>
>>>>> #include <sys/mman.h>
>>>>> #include <sys/syscall.h>
>>>>>
>>>>> int main(void)
>>>>> {
>>>>>             int fd1, fd2;
>>>>>             int pip[2];
>>>>>             struct iovec iov;
>>>>>             char *addr;
>>>>>             int ret;
>>>>>
>>>>>             fd1 = syscall(__NR_memfd_secret, 0);
>>>>>             addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd1, 0);
>>>>>             ftruncate(fd1, 7);
>>>>>             addr[0] = 1; /* fault in page */
>>>
>>> Here the page is faulted in and GUP-fast will find it.  It's not in
>>> the kernel page table, but it is in the user page table, which is what
>>> matter for GUP.
>>
>> Trust me, I know the GUP code very well :P
>>
>> gup_pte_range -- GUP fast -- contains:
>>
>> if (unlikely(folio_is_secretmem(folio))) {
>> 	gup_put_folio(folio, 1, flags);
>> 	goto pte_unmap;
>> }
>>
>> So we "should" be rejecting any secretmem folios and fallback to GUP slow.
>>
>>
>> ... we don't check the same in gup_huge_pmd(), but we shouldn't ever see
>> THP in secretmem code.
>>
> 
> Ehm:
> 
> [   29.441405] Secretmem fault: PFN: 1096177
> [   29.442092] GUP-fast: PFN: 1096177
> 
> 
> ... is folio_is_secretmem() broken?
> 
> ... is it something "obvious" like:
> 
> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
> index 35f3a4a8ceb1e..6996f1f53f147 100644
> --- a/include/linux/secretmem.h
> +++ b/include/linux/secretmem.h
> @@ -16,7 +16,7 @@ static inline bool folio_is_secretmem(struct folio *folio)
>            * We know that secretmem pages are not compound and LRU so we can
>            * save a couple of cycles here.
>            */
> -       if (folio_test_large(folio) || !folio_test_lru(folio))
> +       if (folio_test_large(folio) || folio_test_lru(folio))
>                   return false;
>    
>           mapping = (struct address_space *)

... yes, that does the trick!

-- 
Cheers,

David / dhildenb