On Tue, May 02, 2023 at 02:45:01PM -0400, Matthew Rosato wrote: > On 5/2/23 12:34 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. > > > > v7: > > - Fixed very silly bug in writeable_file_mapping_allowed() inverting the > > logic. > > - Removed unnecessary RCU lock code and replaced with adaptation of Peter's > > idea. > > - Removed unnecessary open-coded folio_test_anon() in > > folio_longterm_write_pin_allowed() and restructured to generally permit > > NULL folio_mapping(). > > > > FWIW, I realize you are planning another respin, but I went and tried this version out on s390 -- Now when using a memory backend file and vfio-pci on s390 I see vfio_pin_pages_remote failing consistently. However, the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable will still return positive. > Hey thanks very much for checking that :) This version will unconditionally apply the retriction to non-FOLL_LONGTERM by mistake (ugh) but vfio_pin_pages_remote() does seem to be setting FOLL_LONGTERM anyway so this seems a legitimate test. Interesting the _fast() variant succeeds... David, Jason et al. can speak more to the ins and outs of these virtualisation cases which I am not so familiar with, but I wonder if we do need a flag to provide an exception for VFIO.
