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b=omgaDKgJreyKgJLexcoMHjOwqx1MK/e5dP/JDVQ8JtQ92CSUv32+PjR1ugJgqVXTV 7cRH6SFc4bHjO674c8wHy7txXJLVzI+PkxvA5ew5S+KxPUXU/uM0pxQ69d0kpF6I68 FWqtuMpw2veFcwIldmiBnTKW0CqWhdWS/EEwfyflg1v1i0abgs33V1gSbh7tywyUSl ompLEcETY+wJOAbmFhWk9GXr3M5Y60o97QZRUfhu16xbWBtC5PsrwDWhIvw9sGEmXs wzSaEz3386mSZozz9ZBpsJx4uBZi4LKdmbCjMMDsDQz27z3HEmWUNj7X2cD0p6spmV R9He2QM6c+D6w== Message-ID: <245e6969-f45b-4d00-bf34-83da529cd3c1@kernel.org> Date: Mon, 8 Dec 2025 20:29:19 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/page_alloc: make percpu_pagelist_high_fraction reads lock-free To: Aboorva Devarajan , Andrew Morton , gourry@gourry.net, mhocko@suse.com Cc: vbabka@suse.cz, surenb@google.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251201060009.1420792-1-aboorvad@linux.ibm.com> <20251201094112.07eb1e588b6da2ee70c4641d@linux-foundation.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: ijgbetqbdhx6n1wh7fgb85cky41oihms X-Rspamd-Queue-Id: A4BB214000C X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1765222165-706563 X-HE-Meta: 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 vrKSent0 TLcyIKi2PtHp5tBA1Aytd7pNpfba79IfhNIyd1NLG7sky2sN67MXHWSpqU8vtaEJJmauYEQzXBCy0Ppxx2p7GfqpQFD609nDDIBGQGl/G1vk+0SBEMG1n2o/m9xVNF0pw3UISwWGbfCad8ZfM8HRronlWJ3KuO581FD4zN+tN55IRKIlt9j5ZRSVOSdB4qpplPWDOYcLSCqItLWfds+B6TXib5DG90aDJtJUZWwG69/8pI4j5o89u5vss8kYvlmpeb/tQQKnz8adWw2p6ugLRl5pXVnavxD1a1+Hv+IxBmxpU9ZsFUTELAUQisYucmnVbYB5WKJstBaDEc0/ddYE1JSTQmKqHh7o5uiix X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 12/8/25 18:30, Aboorva Devarajan wrote: > On Mon, 2025-12-01 at 09:41 -0800, Andrew Morton wrote: >> On Mon,  1 Dec 2025 11:30:09 +0530 Aboorva Devarajan wrote: >> >>> When page isolation loops indefinitely during memory offline, reading >>> /proc/sys/vm/percpu_pagelist_high_fraction blocks on pcp_batch_high_lock, >>> causing hung task warnings. >> >> That's pretty bad behavior. >> >> I wonder if there are other problems which can be caused by this >> lengthy hold time. >> >> It would be better to address the lengthy hold time rather that having >> to work around it in one impacted site. > > > Sorry for the delayed response, I spent some time recreating this issue. > > > I've encountered this lengthy hold time several times during memory hot-unplug, with > operations hanging indefinitely (20+ hours). It occurs intermittently, and it has > different failure signatures, here's one example where isolation fails on a single > slab page continuously: > > .. > [83310.373699] page dumped because: isolation failed > [83310.373704] failed to isolate pfn 4dc68 > [83310.373708] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68 > [83310.373714] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff) > [83310.373722] page_type: f5(slab) > [83310.373727] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122 > [83310.373735] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000 > [83310.373741] page dumped because: isolation failed > [83310.373749] failed to isolate pfn 4dc68 > [83310.373753] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68 > [83310.373760] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff) > [83310.373767] page_type: f5(slab) > [83310.373770] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122 > [83310.373774] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000 > [83310.373778] page dumped because: isolation failed > [83310.373788] failed to isolate pfn 4dc68 > [83310.373791] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68 > [83310.373794] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff) > [83310.373797] page_type: f5(slab) > [83310.373799] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122 > [83310.373803] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000 > [83310.373809] page dumped because: isolation failed > [83315.383370] do_migrate_range: 1098409 callbacks suppressed > [83315.383377] failed to isolate pfn 4dc68 > [83315.383406] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68 > [83315.383411] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff) > [83315.383416] page_type: f5(slab) > [83315.383420] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122 > [83315.383423] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000 > [83315.383426] page dumped because: isolation failed > [83315.383431] failed to isolate pfn 4dc68 > [83315.383433] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4dc68 > [83315.383442] flags: 0x23ffffe00000000(node=2|zone=0|lastcpupid=0x1fffff) > [83315.383448] page_type: f5(slab) > [83315.383454] raw: 023ffffe00000000 c0000028e001fa00 5deadbeef0000100 5deadbeef0000122 > [83315.383462] raw: 0000000000000000 0000000001e101e1 00000002f5000000 0000000000000000 > [83315.383470] page dumped because: isolation failed When starting memory offlining we do a racy check whether memory offlining will succeed in has_unmovable_pages(). It's racy because briefly after the check, a kernel page could get allocated, before we isolate all the free pages. I assume that's what happened here. But even performing another has_unmovable_pages() check after isolating the free pages will not catch all cases: in particular movable pages that are longterm-pinned. -- Cheers David