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Date: Fri, 22 Mar 2024 22:33:45 +0100
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Subject: Re: BUG: unable to handle kernel paging request in fuse_copy_do
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From: David Hildenbrand <david@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: xingwei lee <xrivendell7@gmail.com>, linux-fsdevel@vger.kernel.org,
 linux-kernel@vger.kernel.org, samsun1006219@gmail.com,
 syzkaller-bugs@googlegroups.com, linux-mm <linux-mm@kvack.org>,
 Mike Rapoport <rppt@kernel.org>
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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 *)


-- 
Cheers,

David / dhildenb