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GUP-fast appears to be one such lockless pgtable w= alker. >>>>> >>>>> Could you please explain how bad things might happen ? >>>> >>>> See 2 places where pmdp_get_lockless() is called in gup.c, without t= he PTL. >>>> These could both return the swap pte for which pmd_mkinvalid() has b= een called. >>>> In both cases, this would lead to the pmd_present() check eroneously= returning >>>> true, eventually causing incorrect interpretation of the pte fields.= e.g.: >>>> >>>> gup_pmd_range() >>>> pmd_t pmd =3D pmdp_get_lockless(pmdp); >>>> gup_huge_pmd(pmd, ...) >>>> page =3D nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SH= IFT); >>>> >>>> page is guff. >>>> >>>> Let me know what you think! >> >> Add JohnH to check GUP code. > > Ryan is correct about this behavior. > > By the way, remember that gup is not the only lockless page table > walker: there is also the CPU hardware itself, which inconveniently > refuses to bother with taking page table locks. :) > > So if we have code that can make a non-present PTE appear to be present= > to any of these page walkers, whether software or hardware, it's a > definitely Not Good and will lead directly to bugs. This issue does not bother hardware, because the PTE_VALID/PMD_SECT_VALID= is always unset and hardware always sees this PMD as invalid. It is a pur= e software issue, since for THP splitting, we do not want hardware to acces= s the page but still allow kernel to user pmd_page() to get the pfn, so pmd_present() returns true even if PTE_VALID/PMD_SECT_VALID is unset by setting and checking PMD_PRESENT_INVALID bit. pmd_mkinvalid() sets PMD_PRESENT_INVALID, turning a migration entry from !pmd_present() to pmd_present(), while it is always a invalid PMD to hardware. > > Since I had to study this patch and discussion a bit in order to > respond, I'll go ahead and also reply to the original patch with review= > comments. > > > thanks, > -- = > John Hubbard > NVIDIA -- Best Regards, Yan, Zi --=_MailMate_1FD05359-9965-47F6-8621-EF7035B67727_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEE6rR4j8RuQ2XmaZol4n+egRQHKFQFAmYtFK4PHHppeUBudmlk aWEuY29tAAoJEOJ/noEUByhU7JYP/RIZ/7hML6LeEGHYZEKBJ/7xNKLghDODAmtT LiFkOtgs6LusbevS4837wxNm4bigQUfiZ9vveWsc2HXeiJEzUqyiKShPny2B8yTW 7MAx79JqTeA/zl0RDYonXrYVXIrAKEab0wwZdlPksaZLwAJ2FGZZAt0LdOKOC7vd cR2if2BSPWvivwQga6DAJNLZUEqOxJ1VdFvNlpwRSPJUHQRlra2rzJ4tsroyKB6y VS6cJqVv33aX/EpS7llqZ0CM4LY+FZMLJeM9UNS+nWQTdDUE3mHugerToVAe6Iqx qQDcsMqarpBTq5zr+wA2XIsbuYwoKHup5yVj2nhGv/5W4cckvNGYm3E+GCmMBiSU mbP11JKnCsa1lYZnHP6+/2ol7X0eBWSnZtVSzqjSL00fT3BfXoIy5z/wm4H6ddKV DIgviefFej2yVCINL7sUuTQGH2AUSKaf8Ve2WzMHHr+9rPUV10FWVi2QDC1p4WMH V2c3ZEp2izTAEMs4w/6hOSVZkyVGOFV4Y2f11BPgKsbE+2pulu7cvfxp396qBKgb YlPsrUIHrndrFDj1SaqSUFutB1eTirLXU/850YUInMAR0/fpYdSGhQTCiH52kgBs gI9S0X6I/Z/w9W5sLxhFEH/Azej24CP1cnAXFWiMmyAE79ODvoQ0QMv2enfacYLA 7rSjicdT =zPD1 -----END PGP SIGNATURE----- --=_MailMate_1FD05359-9965-47F6-8621-EF7035B67727_=--