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Date: Tue, 8 Sep 2020 11:26:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>,
Pavel Tatashin <pasha.tatashin@...een.com>
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
linux-mm@...ck.org, osalvador@...e.de, richard.weiyang@...il.com,
vbabka@...e.cz, rientjes@...gle.com
Subject: Re: [PATCH v3 2/2] mm: drain per-cpu pages outside of
isolate_migratepages_range
>> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
>
> I believe we should be going an opposite direction and define a more
> understandable and usable semantic for start_isolate_page_range. We do
> not want callers to scratch their heads to call all caches they might
> need to flush.
I still prefer temporarily disabling PCP, or using a different mechanism
to teach PCP to no work on isolated pageblocks (while the latter seems
to be harder to achieve). I can spot that Vlastimil already tried to
implement it - I'll have a look shortly.
Having that said, this patch improves the situation in case we cannot
get temporarily disabling of PCP implemented - instead of flushing at
random place, we flush at less random places :)
But it is far from ideal. It's just another of the special cases that
developed over time and instead of properly fixing it, we work around it.
Note: I think the flush semantics of drain_all_pages() is very different
from lru_add_drain_all(). The latter does not mess with isolated page
that have just been freed (as it's not part of the core buddy / buddy
extension).
>
>> ---
>> mm/memory_hotplug.c | 15 +--------------
>> mm/page_alloc.c | 2 ++
>> mm/page_isolation.c | 40 ++++++++++++----------------------------
>> 3 files changed, 15 insertions(+), 42 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index b11a269e2356..5a2ed1a94555 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1536,6 +1536,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>> }
>>
>> do {
>> + drain_all_pages(zone);
>> pfn = start_pfn;
>> do {
>> if (signal_pending(current)) {
>> @@ -1575,20 +1576,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>> /* check again */
>> ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
>> NULL, check_pages_isolated_cb);
>> - /*
>> - * per-cpu pages are drained in start_isolate_page_range, but if
>> - * there are still pages that are not free, make sure that we
>> - * drain again, because when we isolated range we might
>> - * have raced with another thread that was adding pages to pcp
>> - * list.
>> - *
>> - * Forward progress should be still guaranteed because
>> - * pages on the pcp list can only belong to MOVABLE_ZONE
>> - * because has_unmovable_pages explicitly checks for
>> - * PageBuddy on freed pages on other zones.
>> - */
>> - if (ret)
>> - drain_all_pages(zone);
>> } while (ret);
>>
>> /* Ok, all of our target is isolated.
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index fab5e97dc9ca..6d6a501a103e 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -8462,6 +8462,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>> if (ret < 0)
>> return ret;
>>
>> + drain_all_pages(cc.zone);
>> +
>> /*
>> * In case of -EBUSY, we'd like to know which page causes problem.
>> * So, just fall through. test_pages_isolated() has a tracepoint
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index 63a3db10a8c0..8dfa6c6c668d 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -19,8 +19,8 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>> {
>> struct page *unmovable = NULL;
>> struct zone *zone;
>> - unsigned long flags;
>> - int ret = -EBUSY;
>> + unsigned long flags, nr_pages;
>> + int ret = -EBUSY, mt;
>>
>> zone = page_zone(page);
>>
>> @@ -39,24 +39,18 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>> * We just check MOVABLE pages.
>> */
>> unmovable = has_unmovable_pages(zone, page, migratetype, isol_flags);
>> - if (!unmovable) {
>> - unsigned long nr_pages;
>> - int mt = get_pageblock_migratetype(page);
>> -
>> - set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>> - zone->nr_isolate_pageblock++;
>> - nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE,
>> - NULL);
>> -
>> - __mod_zone_freepage_state(zone, -nr_pages, mt);
>> - ret = 0;
>> - }
>> + if (unmovable)
>> + goto out;
>>
>> + mt = get_pageblock_migratetype(page);
>> + set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>> + zone->nr_isolate_pageblock++;
>> + nr_pages = move_freepages_block(zone, page, MIGRATE_ISOLATE, NULL);
>> + __mod_zone_freepage_state(zone, -nr_pages, mt);
>> + ret = 0;
>> out:
>> spin_unlock_irqrestore(&zone->lock, flags);
>> - if (!ret) {
>> - drain_all_pages(zone);
>> - } else {
>> + if (ret) {
>> WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
>>
There are patches in -mm/-next by me that already cleanup/reshuffle that
code heavily, so better just keep the changes minimal or built up on top
of -next.
>> if ((isol_flags & REPORT_FAILURE) && unmovable)
>> @@ -170,14 +164,6 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>> * pageblocks we may have modified and return -EBUSY to caller. This
>> * prevents two threads from simultaneously working on overlapping ranges.
>> *
>> - * Please note that there is no strong synchronization with the page allocator
>> - * either. Pages might be freed while their page blocks are marked ISOLATED.
>> - * In some cases pages might still end up on pcp lists and that would allow
>> - * for their allocation even when they are in fact isolated already. Depending
>> - * on how strong of a guarantee the caller needs drain_all_pages might be needed
>> - * (e.g. __offline_pages will need to call it after check for isolated range for
>> - * a next retry).
>> - *
>> * Return: the number of isolated pageblocks on success and -EBUSY if any part
>> * of range cannot be isolated.
>> */
>> @@ -192,9 +178,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>> BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
>> BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
>>
>> - for (pfn = start_pfn;
>> - pfn < end_pfn;
>> - pfn += pageblock_nr_pages) {
>> + for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
unnecessary code churn.
>> page = __first_valid_page(pfn, pageblock_nr_pages);
>> if (page) {
>> if (set_migratetype_isolate(page, migratetype, flags)) {
>> --
>> 2.25.1
>>
>
--
Thanks,
David / dhildenb
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