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.
