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charset=utf-8 > On Mar 10, 2026, at 4:19=E2=80=AFPM, David Hildenbrand (Arm) = wrote: >=20 > !-------------------------------------------------------------------| > This Message Is =46rom an External Sender > This message came from outside your organization. > |-------------------------------------------------------------------! >>=20 >> I was previously testing on 6.12 and didn=E2=80=99t see any changes = to vfio-pci or >> pagewalk.c which prompted me to check whether I could reproduce the >> bug in a more recent kernel. =20 >>=20 >> However, when I tried to reproduce the bug on 7.0-rc2 (after adding = some >> tracing to get a clearer picture of the sequence of events) it = doesn=E2=80=99t happen. >> The VFIO DMA set operation is much faster on 7.0, so possibly the = race=20 >> window is too small for it to occur in reasonable time. >=20 > Interesting. You could try adding a delay to a test kernel to see if = you > can still provoke it. >=20 > There is the slight possibility that something else fixed the race for > your reproducer by "accident". >=20 > [...] Could be that the race window was made a lot smaller by [1], I see that = the occurrence of the race also drops significantly already when I=E2=80=99m just = adding some extra trace logs in the VFIO DMA set functions. [=E2=80=A6] >=20 > Just to double-check: is that expected? >=20 > I wonder why "-EINVAL" would be returned here. Do you know? >=20 I don=E2=80=99t think it=E2=80=99s expected, but at least it=E2=80=99s = an error that can be caught in userspace. I=E2=80=99ll do a bit more digging into why and where that originates, = but I think that=E2=80=99ll get a patch in vfio. The -EINVAL originates from: vfio_dma_do_map -> vfio_pin_map_dma -> vfio_pin_pages_remote -> vaddr_get_pfns -> pin_user_pages_remote (mm/gup.c) Possibly that=E2=80=99s also the origin of the concurrent PUD = modification that requires the retry in the walker in this patch. >>=20 >> For my own understanding, why is this patch preferred over: >> - if (pud_none(*pud)) >> + if (pud_none(*pud) || pud_leaf(*pud)) >> in the walk_pud_range function? >=20 > It might currently work for PUDs, but as soon as we have non-present = PUD > entries (like migration entries) the code could become shaky: = pud_leaf() > is only guaranteed to yield the right result if pud_present() is true. >=20 > So I decided to instead make walk_pud_range() look more similar to > walk_pmd_range(), which is quite helpful for spotting actual = differences > in the logic. >=20 >>=20 >> I do think moving the check to walk_pmd_range is a more clear on the = code=E2=80=99s intent and >> personally prefer the code there, but I don=E2=80=99t see why this = check is removing the possibility >> of a race after the (!pud_present(pudval) || pud_leaf(pudval)) check, = as to me it looks >> like the PMD entry was possible to disappear between the splitting = and this check? >=20 > I distilled that in the comment: PMD page tables cannot/are not > reclaimed. So once you see a PMD page table, it's not going anywhere > while you hold relevant locks (mmap_lock or VMA lock). >=20 > Only PMD leaf entries can get zapped any time and PMD none entries can > get populated any time. But not PMD page tables. Gotcha, thanks! >=20 >>=20 >> Anyways, regardless, this patch resolves the bug and looks good to me = - what=E2=80=99s the=20 >> course of action as we probably want to backport this to earlier = kernels as well. Shall >> I send in a new PATCH without cover letter and take it from there? >=20 > Right, I think you should: >=20 > (1) rework the patch description to incorporate the essential stuff = from > the cover letter > (2) Identify and add Fixes: tag and Cc: stable > (3) Document that we are reworking the code to mimic what we do in > walk_pmd_range(), to have less inconsistency on the core logic > (4) Document why you think the reproducer fails on newer kernels. (or > best try to get it reproduced by adding some delays in the code) > (5) Clarify that only PUD handling are prone to the race and that PMDs > are fine (and point out why) > (6) Use a patch subject like "mm/pagewalk: fix race between unmapping > and refaulting in walk_pud_range()" >=20 > Once you resend, best to add >=20 > Co-developed-by: David Hildenbrand (Arm) > Signed-off-by: David Hildenbrand (Arm) >=20 > Above your SOB. >=20 > To get something like: >=20 > Co-developed-by: David Hildenbrand (Arm) > Signed-off-by: David Hildenbrand (Arm) > Signed-off-by: Max Boone >=20 > Note that the existing >=20 > Signed-off-by: Max Tottenham >=20 > Is weird, as Max Tottenham did not send out this patch. If he was > involved in the development, you should either make him >=20 > Suggested-by: >=20 > Or > Debugged-by: >=20 > Or > Co-developed-by: + Signed-off-by: >=20 > See Documentation/process/submitting-patches.rst >=20 >=20 > Let me know if you have any questions :) >=20 Will do, thanks a lot! > --=20 > Cheers, >=20 > David [1] = https://lore.kernel.org/all/20250814064714.56485-1-lizhe.67@bytedance.com/= --Apple-Mail=_C4E92512-2433-4D58-BCE5-E5CCE563A1B4 Content-Disposition: attachment; filename="smime.p7s" Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgEFADCABgkqhkiG9w0BBwEAAKCCCcow ggShMIIESKADAgECAhMxAAAAIa0XYPGypwcKAAAAAAAhMAoGCCqGSM49BAMCMD8xITAfBgNVBAoT GEFrYW1haSBUZWNobm9sb2dpZXMgSW5jLjEaMBgGA1UEAxMRQWthbWFpQ29ycFJvb3QtRzEwHhcN MjQxMTIxMTgzNzUyWhcNMzQxMTIxMTg0NzUyWjA8MSEwHwYDVQQKExhBa2FtYWkgVGVjaG5vbG9n aWVzIEluYy4xFzAVBgNVBAMTDkFrYW1haUNsaWVudENBMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcD QgAEjkdeMHsSTytADJ7eJ+O+5mpBfm9hVC6Cg9Wf+ER8HXid3E68IHjcCTNFSiezqYclAnIalS1I cl6hRFZiacQkd6OCAyQwggMgMBIGCSsGAQQBgjcVAQQFAgMBAAEwIwYJKwYBBAGCNxUCBBYEFOa0 4dX2BYnqjkbEVEwLgf7BQJ7ZMB0GA1UdDgQWBBS2N+ieDVUAjPmykf1ahsljEXmtXDCBrwYDVR0g BIGnMIGkMIGhBgsqAwSPTgEJCQgBATCBkTBYBggrBgEFBQcCAjBMHkoAQQBrAGEAbQBhAGkAIABD AGUAcgB0AGkAZgBpAGMAYQB0AGUAIABQAHIAYQBjAHQAaQBjAGUAIABTAHQAYQB0AGUAbQBlAG4A dDA1BggrBgEFBQcCARYpaHR0cDovL2FrYW1haWNybC5ha2FtYWkuY29tL0FrYW1haUNQUy5wZGYw bAYDVR0lBGUwYwYIKwYBBQUHAwIGCCsGAQUFBwMEBgorBgEEAYI3FAICBgorBgEEAYI3CgMEBgor BgEEAYI3CgMMBggrBgEFBQcDBwYIKwYBBQUHAwkGCSsGAQQBgjcVBQYKKwYBBAGCNxQCATAZBgkr BgEEAYI3FAIEDB4KAFMAdQBiAEMAQTALBgNVHQ8EBAMCAYYwDwYDVR0TAQH/BAUwAwEB/zAfBgNV HSMEGDAWgBStAYfq3FmusRM5lU0PV6Akhot7vTCBgAYDVR0fBHkwdzB1oHOgcYYxaHR0cDovL2Fr YW1haWNybC5ha2FtYWkuY29tL0FrYW1haUNvcnBSb290LUcxLmNybIY8aHR0cDovL2FrYW1haWNy bC5kZncwMS5jb3JwLmFrYW1haS5jb20vQWthbWFpQ29ycFJvb3QtRzEuY3JsMIHIBggrBgEFBQcB AQSBuzCBuDA9BggrBgEFBQcwAoYxaHR0cDovL2FrYW1haWNybC5ha2FtYWkuY29tL0FrYW1haUNv cnBSb290LUcxLmNydDBIBggrBgEFBQcwAoY8aHR0cDovL2FrYW1haWNybC5kZncwMS5jb3JwLmFr YW1haS5jb20vQWthbWFpQ29ycFJvb3QtRzEuY3J0MC0GCCsGAQUFBzABhiFodHRwOi8vYWthbWFp b2NzcC5ha2FtYWkuY29tL29jc3AwCgYIKoZIzj0EAwIDRwAwRAIgaUoJ7eBk/qNcBVTJW5NC4NsO 6j4/6zQoKeKgOpeiXQUCIGkbSN83n1mMURZIK92KFRtn2X1nrZ7rcNuAQD5bvH1bMIIFITCCBMig AwIBAgITFwALOJfLRtbGzZc1dwABAAs4lzAKBggqhkjOPQQDAjA8MSEwHwYDVQQKExhBa2FtYWkg VGVjaG5vbG9naWVzIEluYy4xFzAVBgNVBAMTDkFrYW1haUNsaWVudENBMB4XDTI1MDgyODA3NTYy OVoXDTI3MDgyODA3NTYyOVowTjEZMBcGA1UECxMQTWFjQm9vayBQcm8tNDZZVDEPMA0GA1UEAxMG bWJvb25lMSAwHgYJKoZIhvcNAQkBFhFtYm9vbmVAYWthbWFpLmNvbTCCASIwDQYJKoZIhvcNAQEB BQADggEPADCCAQoCggEBAOX+npfSrX/rwhOySq6aejQMUVslPFpNvXdEnmMlnEjR95gq0Ygp+wQc Sde+JGBpGHsPMzHT1Nd3V1acm4cW1WB1aRqJOMfSLifg6SLkq2EM9WsftEiA1G4BT4UP0PFZY2Os 6TXvebAuVg6LwhB417rEJ2kuS/DKpiG8trAVDR6Uy9vbSMBp6iIewBc9r0CjW8l1zgRr+uQpXEUP mF2BV0l3Qo5r0nhPqTWR9oAX4/oTqnhbEhQ3tOFYTjzO1K9DdzX8mVggVSZz/M0v0gtkZVvO4B1t 3Sh+1lla5eMY4hlVHW1/FKqMe4EMXmDH7goTEuXPpelJiNRdBh7ud7xNNFUCAwEAAaOCAsowggLG MAsGA1UdDwQEAwIHgDApBgNVHSUEIjAgBggrBgEFBQcDAgYIKwYBBQUHAwQGCisGAQQBgjcKAwQw HQYDVR0OBBYEFO0y/xWMpkyOUMuNKmuzNtjXpdtRMEQGA1UdEQQ9MDugJgYKKwYBBAGCNxQCA6AY DBZtYm9vbmVAY29ycC5ha2FtYWkuY29tgRFtYm9vbmVAYWthbWFpLmNvbTAfBgNVHSMEGDAWgBS2 N+ieDVUAjPmykf1ahsljEXmtXDCBgAYDVR0fBHkwdzB1oHOgcYYxaHR0cDovL2FrYW1haWNybC5h a2FtYWkuY29tL0FrYW1haUNsaWVudENBKDEpLmNybIY8aHR0cDovL2FrYW1haWNybC5kZncwMS5j b3JwLmFrYW1haS5jb20vQWthbWFpQ2xpZW50Q0EoMSkuY3JsMIHIBggrBgEFBQcBAQSBuzCBuDA9 BggrBgEFBQcwAoYxaHR0cDovL2FrYW1haWNybC5ha2FtYWkuY29tL0FrYW1haUNsaWVudENBKDEp LmNydDBIBggrBgEFBQcwAoY8aHR0cDovL2FrYW1haWNybC5kZncwMS5jb3JwLmFrYW1haS5jb20v QWthbWFpQ2xpZW50Q0EoMSkuY3J0MC0GCCsGAQUFBzABhiFodHRwOi8vYWthbWFpb2NzcC5ha2Ft YWkuY29tL29jc3AwOwYJKwYBBAGCNxUHBC4wLAYkKwYBBAGCNxUIgs7lOoe41C2BhYsHouMhhtIP gUmFpcMQmtV/AgFkAgFTMDUGCSsGAQQBgjcVCgQoMCYwCgYIKwYBBQUHAwIwCgYIKwYBBQUHAwQw DAYKKwYBBAGCNwoDBDBEBgkqhkiG9w0BCQ8ENzA1MA4GCCqGSIb3DQMCAgIAgDAOBggqhkiG9w0D BAICAIAwBwYFKw4DAgcwCgYIKoZIhvcNAwcwCgYIKoZIzj0EAwIDRwAwRAIgD5UL4MI1RXeg64RR kifZAeItCnkZ4ecrqSEGpLcXV+ICIAdB9vZdM1WGxtag0rlqG0j0FBrCWixC0cdHNpFrqNx/MYIB 6TCCAeUCAQEwUzA8MSEwHwYDVQQKExhBa2FtYWkgVGVjaG5vbG9naWVzIEluYy4xFzAVBgNVBAMT DkFrYW1haUNsaWVudENBAhMXAAs4l8tG1sbNlzV3AAEACziXMA0GCWCGSAFlAwQCAQUAoGkwGAYJ KoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMjYwMzExMDk0MjQ1WjAvBgkq hkiG9w0BCQQxIgQgeusbe69mlHKMlSfoSznhTfD1D86TwpMfQXcu0mKPpvEwDQYJKoZIhvcNAQEL BQAEggEAO2bM25GtW+C2+9G4Z0J8hauauikb6xSc7RzALuVXuJpT5uZc6L1bGDgC96gydWsjz500 4ZaQDKXM4HksiJ+HCHNzrEJzWtWWqeXY2UxSO7ppOWpgy09z8crDvyFITUdUo9iOo26VtroA7Oy1 e4mjPiEy4Z5+4lSGdLxBsgSA3skNoYS/UskxjtvmJamw6hrcO7X+LqAFcCEgoAXC1laA7IRO/qiV MYP7tEChXlvLJv8VknT072IfuQAmapewddmPuJBxiG0ZWlxOWBFcRi8UOFzQUO9JOx/Idi3n1h65 yMRCFEhJoJamdkVOcOdtMnosDWH5Bkpr1oQWgizcqRHs8gAAAAAAAA== --Apple-Mail=_C4E92512-2433-4D58-BCE5-E5CCE563A1B4--