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Date:   Thu, 22 Sep 2022 18:20:28 +0530
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Subject: Re: [RFC 0/6] migrate_pages(): batch TLB flushing
Content-Language: en-US
To:     Huang Ying <ying.huang@intel.com>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Zi Yan <ziy@nvidia.com>, Yang Shi <shy828301@gmail.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>
References: <20220921060616.73086-1-ying.huang@intel.com>
From:   Bharata B Rao <bharata@amd.com>
In-Reply-To: <20220921060616.73086-1-ying.huang@intel.com>
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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().

Regards,
Bharata.