Message-ID: <1b34e9a4-83c0-2f44-1457-dd8800b9287a@redhat.com>
Date: Wed, 3 May 2023 09:08:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Rosato <mjrosato@...ux.ibm.com>,
 Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>
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Subject: Re: [PATCH v8 0/3] mm/gup: disallow GUP writing to file-backed
 mappings by default

On 03.05.23 02:31, Matthew Rosato wrote:
> On 5/2/23 6:51 PM, Lorenzo Stoakes wrote:
>> Writing to file-backed mappings which require folio dirty tracking using
>> GUP is a fundamentally broken operation, as kernel write access to GUP
>> mappings do not adhere to the semantics expected by a file system.
>>
>> A GUP caller uses the direct mapping to access the folio, which does not
>> cause write notify to trigger, nor does it enforce that the caller marks
>> the folio dirty.
>>
>> The problem arises when, after an initial write to the folio, writeback
>> results in the folio being cleaned and then the caller, via the GUP
>> interface, writes to the folio again.
>>
>> As a result of the use of this secondary, direct, mapping to the folio no
>> write notify will occur, and if the caller does mark the folio dirty, this
>> will be done so unexpectedly.
>>
>> For example, consider the following scenario:-
>>
>> 1. A folio is written to via GUP which write-faults the memory, notifying
>>     the file system and dirtying the folio.
>> 2. Later, writeback is triggered, resulting in the folio being cleaned and
>>     the PTE being marked read-only.
>> 3. The GUP caller writes to the folio, as it is mapped read/write via the
>>     direct mapping.
>> 4. The GUP caller, now done with the page, unpins it and sets it dirty
>>     (though it does not have to).
>>
>> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As
>> pin_user_pages_fast_only() does not exist, we can rely on a slightly
>> imperfect whitelisting in the PUP-fast case and fall back to the slow case
>> should this fail.
>>
>> v8:
>> - Fixed typo writeable -> writable.
>> - Fixed bug in writable_file_mapping_allowed() - must check combination of
>>    FOLL_PIN AND FOLL_LONGTERM not either/or.
>> - Updated vma_needs_dirty_tracking() to include write/shared to account for
>>    MAP_PRIVATE mappings.
>> - Move to open-coding the checks in folio_pin_allowed() so we can
>>    READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to
>>    account for fact we now check flags here.
>> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than
>>    anon. Defer to slow path.
>> - Perform GUP-fast check _after_ the lowest page table level is confirmed to
>>    be stable.
>> - Updated comments and commit message for final patch as per Jason's
>>    suggestions.
> 
> Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count)

At least it's consistent now ;) And it might be working as expected ...

In v7:
* pin_user_pages_fast() succeeded
* vfio_pin_pages_remote() failed

But also in v7:
* GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file
   mappings
* Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings

In v8:
* pin_user_pages_fast() succeeds
* vfio_pin_pages_remote() succeeds

But also in v8:
* GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file
   mappings
* Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings


I have to speculate, but ... could it be that you are using a private 
mapping?

In QEMU, unfortunately, the default for memory-backend-file is 
"share=off" (private) ... for memory-backend-memfd it is "share=on" 
(shared). The default is stupid ...

If you invoke QEMU manually, can you specify "share=on" for the 
memory-backend-file? I thought libvirt would always default to 
"share=on" for file mappings (everything else doesn't make much sense) 
... but you might have to specify
	<access mode="shared"/>
in addition to
	<source type="file"/>

-- 
Thanks,

David / dhildenb

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