Re: [RFC 5/5] mm, page_alloc: disable pcplists during page isolation

From: David Hildenbrand
Date: Wed Sep 09 2020 - 10:30:49 EST

On 09.09.20 13:44, David Hildenbrand wrote:
> On 09.09.20 13:36, Michal Hocko wrote:
>> On Wed 09-09-20 12:48:54, Vlastimil Babka wrote:
>>> Here's a version that will apply on top of next-20200908. The first 4 patches need no change.
>>> ----8<----
>>> >From 8febc17272b8e8b378e2e5ea5e76b2616f029c5b Mon Sep 17 00:00:00 2001
>>> From: Vlastimil Babka <vbabka@xxxxxxx>
>>> Date: Mon, 7 Sep 2020 17:20:39 +0200
>>> Subject: [PATCH] mm, page_alloc: disable pcplists during page isolation
>>> Page isolation can race with process freeing pages to pcplists in a way that
>>> a page from isolated pageblock can end up on pcplist. This can be fixed by
>>> repeated draining of pcplists, as done by patch "mm/memory_hotplug: drain
>>> per-cpu pages again during memory offline" in [1].
>>> David and Michal would prefer that this race was closed in a way that callers
>>> of page isolation don't need to care about drain. David suggested disabling
>>> pcplists usage completely during page isolation, instead of repeatedly draining
>>> them.
>>> To achieve this without adding special cases in alloc/free fastpath, we can use
>>> the same 'trick' as boot pagesets - when pcp->high is 0, any pcplist addition
>>> will be immediately flushed.
>>> The race can thus be closed by setting pcp->high to 0 and draining pcplists
>>> once in start_isolate_page_range(). The draining will serialize after processes
>>> that already disabled interrupts and read the old value of pcp->high in
>>> free_unref_page_commit(), and processes that have not yet disabled interrupts,
>>> will observe pcp->high == 0 when they are rescheduled, and skip pcplists.
>>> This guarantees no stray pages on pcplists in zones where isolation happens.
>>> We can use the variable zone->nr_isolate_pageblock (protected by zone->lock)
>>> to detect transitions from 0 to 1 (to change pcp->high to 0 and issue drain)
>>> and from 1 to 0 (to restore original pcp->high and batch values cached in
>>> struct zone). We have to avoid external updates to high and batch by taking
>>> pcp_batch_high_lock. To allow multiple isolations in parallel, change this
>>> lock from mutex to rwsem.
>>> For callers that pair start_isolate_page_range() with
>>> undo_isolated_page_range() properly, this is transparent. Currently that's
>>> alloc_contig_range(). __offline_pages() doesn't call undo_isolated_page_range()
>>> in the succes case, so it has to be carful to handle restoring pcp->high and batch
>>> and unlocking pcp_batch_high_lock.
>> I was hoping that it would be possible to have this completely hidden
>> inside start_isolate_page_range code path. If we need some sort of
>> disable_pcp_free/enable_pcp_free then it seems like a better fit to have
>> an explicit API for that (the naming would be obviously different
>> because we do not want to call out pcp free lists). I strongly suspect
>> that only the memory hotplug really cares for this hard guanrantee.
>> alloc_contig_range simply goes with EBUSY.
> There will be different alloc_contig_range() demands in the future: try
> fast (e.g., loads of small CMA allocations) vs. try hard (e.g.,
> virtio-mem). We can add ways to specify that.

A reference to a related discussion regarding the "try fast" use case in
CMA for the future:


David / dhildenb