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It calls pmdp_invalidate() > unconditionally on the pmdp and only determines if it is present or not= > based on the returned old pmd. > > But arm64's pmd_mkinvalid(), called by pmdp_invalidate(), > unconditionally sets the PMD_PRESENT_INVALID flag, which causes future > pmd_present() calls to return true - even for a swap pmd. Therefore any= > lockless pgtable walker could see the migration entry pmd in this state= > and start interpretting the fields (e.g. pmd_pfn()) as if it were > present, leading to BadThings (TM). GUP-fast appears to be one such > lockless pgtable walker. > > While the obvious fix is for core-mm to avoid such calls for non-presen= t > pmds (pmdp_invalidate() will also issue TLBI which is not necessary for= > this case either), all other arches that implement pmd_mkinvalid() do i= t > in such a way that it is robust to being called with a non-present pmd.= > So it is simpler and safer to make arm64 robust too. This approach mean= s > we can even add tests to debug_vm_pgtable.c to validate the required > behaviour. > > This is a theoretical bug found during code review. I don't have any > test case to trigger it in practice. > > Cc: stable@vger.kernel.org > Fixes: 53fa117bb33c ("arm64/mm: Enable THP migration") > Signed-off-by: Ryan Roberts > --- > > Hi all, > > v1 of this fix [1] took the approach of fixing core-mm to never call > pmdp_invalidate() on a non-present pmd. But Zi Yan highlighted that onl= y arm64 > suffers this problem; all other arches are robust. So his suggestion wa= s to > instead make arm64 robust in the same way and add tests to validate it.= Despite > my stated reservations in the context of the v1 discussion, having thou= ght on it > for a bit, I now agree with Zi Yan. Hence this post. > > Andrew has v1 in mm-unstable at the moment, so probably the best thing = to do is > remove it from there and have this go in through the arm64 tree? Assumi= ng there > is agreement that this approach is right one. > > This applies on top of v6.9-rc5. Passes all the mm selftests on arm64. > > [1] https://lore.kernel.org/linux-mm/20240425170704.3379492-1-ryan.robe= rts@arm.com/ > > Thanks, > Ryan > > > arch/arm64/include/asm/pgtable.h | 12 +++++-- > mm/debug_vm_pgtable.c | 61 ++++++++++++++++++++++++++++++++= > 2 files changed, 71 insertions(+), 2 deletions(-) > LGTM. Thanks. Feel free to add Reviewed-by: Zi Yan . -- Best Regards, Yan, Zi --=_MailMate_D4EB5EDB-B3B5-4372-A6AA-1E40E32999F4_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmYxB4YPHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhUX6wP/RiN1STcOZKXdHM57ttW0ZL/Y05QJai6BiO+ mGZ4HZ33Lc2tzulZLkZz1dNScktvDj3MRBBj7d6wivCsZT/uiq9cXiVOZF2QlsJ4 k/5tIJxoH1e98JjNAakTmlsC6xZPv0tYI0fsQdCyNeh41PTaMkadNAyLrb4tSuFR rjgRwEWSWV3adAngzKNn479NWYpqyTXVLcU9LBIp/uQj2vQ5lC/5Vb7yd9eNMOk/ ls5GUwN5xFlrjVg22cY7QDUzQcorNLmBNQInUitj5visvHzJPcw2p8Lwsbgl6g7z WqDI/yn+yF+MsyE/DgE7kn9XGWiOMnjUd2qR7lMLbH1Glb9joCaM9YepgaennMdx eF65hTGtYoTptWvLLUAMhLmteR6fsAAbEOONc/oTbFPtiocGHWiueSoVlx/iwNyr U4lWdxO9U3c+hT1VwLPJEXS1ZR7t++IqL0BrbxvcOUk7gtbMUzUC9efzQIuzJJhx 0hg8uKzPEFwDctGLdv+EGYj6Q0sKyD0kHTILDixUpntH5mNp6OWMDw9dnP3yZfly 4QEAYy/0IrFgAaaDqaT1pUu+kG6PqwgmKN53Dva3p8H79PnWULxpOuLGlGmtDTNh vm3YRNRynP/tB9SXxTtzSobYN4l3I6B0Mqmh4ZzrVmaba4XQTJtsaz6M2qhpmjQy 7XtSeH/Y =fW62 -----END PGP SIGNATURE----- --=_MailMate_D4EB5EDB-B3B5-4372-A6AA-1E40E32999F4_=--