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    SubjectRe: [RFC 0/6] migrate_pages(): batch TLB flushing
    Date
    Bharata B Rao <bharata@amd.com> writes:

    > On 9/21/2022 11:36 AM, Huang Ying wrote:
    >> From: "Huang, Ying" <ying.huang@intel.com>
    >>
    >> Now, migrate_pages() migrate pages one by one, like the fake code as
    >> follows,
    >>
    >> for each page
    >> unmap
    >> flush TLB
    >> copy
    >> restore map
    >>
    >> If multiple pages are passed to migrate_pages(), there are
    >> opportunities to batch the TLB flushing and copying. That is, we can
    >> change the code to something as follows,
    >>
    >> for each page
    >> unmap
    >> for each page
    >> flush TLB
    >> for each page
    >> copy
    >> for each page
    >> restore map
    >>
    >> The total number of TLB flushing IPI can be reduced considerably. And
    >> we may use some hardware accelerator such as DSA to accelerate the
    >> page copying.
    >>
    >> So in this patch, we refactor the migrate_pages() implementation and
    >> implement the TLB flushing batching. Base on this, hardware
    >> accelerated page copying can be implemented.
    >>
    >> If too many pages are passed to migrate_pages(), in the naive batched
    >> implementation, we may unmap too many pages at the same time. The
    >> possibility for a task to wait for the migrated pages to be mapped
    >> again increases. So the latency may be hurt. To deal with this
    >> issue, the max number of pages be unmapped in batch is restricted to
    >> no more than HPAGE_PMD_NR. That is, the influence is at the same
    >> level of THP migration.
    >
    > Thanks for the patchset. I find it hitting the following BUG() when
    > running mmtests/autonumabench:
    >
    > kernel BUG at mm/migrate.c:2432!
    > invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
    > CPU: 7 PID: 7150 Comm: numa01 Not tainted 6.0.0-rc5+ #171
    > Hardware name: Dell Inc. PowerEdge R6525/024PW1, BIOS 2.5.6 10/06/2021
    > RIP: 0010:migrate_misplaced_page+0x670/0x830
    > Code: 36 48 8b 3c c5 e0 7a 19 8d e8 dc 10 f7 ff 4c 89 e7 e8 f4 43 f5 ff 8b 55 bc 85 d2 75 6f 48 8b 45 c0 4c 39 e8 0f 84 b0 fb ff ff <0f> 0b 48 8b 7d 90 e9 ec fc ff ff 48 83 e8 01 e9 48 fa ff ff 48 83
    > RSP: 0000:ffffb1b29ec3fd38 EFLAGS: 00010202
    > RAX: ffffe946460f8248 RBX: 0000000000000001 RCX: ffffe946460f8248
    > RDX: 0000000000000000 RSI: ffffe946460f8248 RDI: ffffb1b29ec3fce0
    > RBP: ffffb1b29ec3fda8 R08: 0000000000000000 R09: 0000000000000005
    > R10: 0000000000000001 R11: 0000000000000000 R12: ffffe946460f8240
    > R13: ffffb1b29ec3fd68 R14: 0000000000000001 R15: ffff9698beed5000
    > FS: 00007fcc31fee640(0000) GS:ffff9697b0000000(0000) knlGS:0000000000000000
    > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    > CR2: 00007fcc3a3a5000 CR3: 000000016e89c002 CR4: 0000000000770ee0
    > PKRU: 55555554
    > Call Trace:
    > <TASK>
    > __handle_mm_fault+0xb87/0xff0
    > handle_mm_fault+0x126/0x3c0
    > do_user_addr_fault+0x1ed/0x690
    > exc_page_fault+0x84/0x2c0
    > asm_exc_page_fault+0x27/0x30
    > RIP: 0033:0x7fccfa1a1180
    > Code: 81 fa 80 00 00 00 76 d2 c5 fe 7f 40 40 c5 fe 7f 40 60 48 83 c7 80 48 81 fa 00 01 00 00 76 2b 48 8d 90 80 00 00 00 48 83 e2 c0 <c5> fd 7f 02 c5 fd 7f 42 20 c5 fd 7f 42 40 c5 fd 7f 42 60 48 83 ea
    > RSP: 002b:00007fcc31fede38 EFLAGS: 00010283
    > RAX: 00007fcc39fff010 RBX: 000000000000002c RCX: 00007fccfa11ea3d
    > RDX: 00007fcc3a3a5000 RSI: 0000000000000000 RDI: 00007fccf9ffef90
    > RBP: 00007fcc39fff010 R08: 00007fcc31fee640 R09: 00007fcc31fee640
    > R10: 00007ffdecef614f R11: 0000000000000246 R12: 00000000c0000000
    > R13: 0000000000000000 R14: 00007fccfa094850 R15: 00007ffdecef6190
    >
    > This is BUG_ON(!list_empty(&migratepages)) in migrate_misplaced_page().

    Thank you very much for reporting! I haven't reproduced this yet. But
    I will pay special attention to this when develop the next version, even
    if I cannot reproduce this finally.

    Best Regards,
    Huang, Ying

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