Messages in this thread | | | Date | Wed, 3 May 2023 14:53:27 +0200 | Subject | Re: [PATCH v8 0/3] mm/gup: disallow GUP writing to file-backed mappings by default | From | David Hildenbrand <> |
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On 03.05.23 13:25, Matthew Rosato wrote: > On 5/3/23 3:08 AM, David Hildenbrand wrote: >> 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"/> >> > > Ah, there we go. Yes, I was using the default of share=off. When I instead specify share=on, now the pins will fail in both cases. >
Out of curiosity, how does that manifest?
I assume the VM is successfully created and as Linux tries initializing and using the device, we get a bunch of errors inside the VM, correct?
-- Thanks,
David / dhildenb
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