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SubjectRe: [PATCH v18 17/32] mm/compaction: do page isolation first in compaction
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在 2020/9/22 上午7:49, Hugh Dickins 写道:
> On Mon, 24 Aug 2020, Alex Shi wrote:
>
>> Currently, compaction would get the lru_lock and then do page isolation
>> which works fine with pgdat->lru_lock, since any page isoltion would
>> compete for the lru_lock. If we want to change to memcg lru_lock, we
>> have to isolate the page before getting lru_lock, thus isoltion would
>> block page's memcg change which relay on page isoltion too. Then we
>> could safely use per memcg lru_lock later.
>>
>> The new page isolation use previous introduced TestClearPageLRU() +
>> pgdat lru locking which will be changed to memcg lru lock later.
>>
>> Hugh Dickins <hughd@google.com> fixed following bugs in this patch's
>> early version:
>>
>> Fix lots of crashes under compaction load: isolate_migratepages_block()
>> must clean up appropriately when rejecting a page, setting PageLRU again
>> if it had been cleared; and a put_page() after get_page_unless_zero()
>> cannot safely be done while holding locked_lruvec - it may turn out to
>> be the final put_page(), which will take an lruvec lock when PageLRU.
>> And move __isolate_lru_page_prepare back after get_page_unless_zero to
>> make trylock_page() safe:
>> trylock_page() is not safe to use at this time: its setting PG_locked
>> can race with the page being freed or allocated ("Bad page"), and can
>> also erase flags being set by one of those "sole owners" of a freshly
>> allocated page who use non-atomic __SetPageFlag().
>>
>> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
>> Signed-off-by: Hugh Dickins <hughd@google.com>
>> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
>
> Okay, whatever. I was about to say
> Acked-by: Hugh Dickins <hughd@google.com>

Thanks!

> With my signed-off-by there, someone will ask if it should say
> "From: Hugh ..." at the top: no, it should not, this is Alex's patch,
> but I proposed some fixes to it, as you already acknowledged.

I guess you prefer to remove your signed off here, don't you?

>
> A couple of comments below on the mm/vmscan.c part of it.
>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> ---
>> include/linux/swap.h | 2 +-
>> mm/compaction.c | 42 +++++++++++++++++++++++++++++++++---------
>> mm/vmscan.c | 46 ++++++++++++++++++++++++++--------------------
>> 3 files changed, 60 insertions(+), 30 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 43e6b3458f58..550fdfdc3506 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -357,7 +357,7 @@ extern void lru_cache_add_inactive_or_unevictable(struct page *page,
>> extern unsigned long zone_reclaimable_pages(struct zone *zone);
>> extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>> gfp_t gfp_mask, nodemask_t *mask);
>> -extern int __isolate_lru_page(struct page *page, isolate_mode_t mode);
>> +extern int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
>> extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>> unsigned long nr_pages,
>> gfp_t gfp_mask,
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 4e2c66869041..253382d99969 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -887,6 +887,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> if (!valid_page && IS_ALIGNED(low_pfn, pageblock_nr_pages)) {
>> if (!cc->ignore_skip_hint && get_pageblock_skip(page)) {
>> low_pfn = end_pfn;
>> + page = NULL;
>> goto isolate_abort;
>> }
>> valid_page = page;
>> @@ -968,6 +969,21 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
>> goto isolate_fail;
>>
>> + /*
>> + * Be careful not to clear PageLRU until after we're
>> + * sure the page is not being freed elsewhere -- the
>> + * page release code relies on it.
>> + */
>> + if (unlikely(!get_page_unless_zero(page)))
>> + goto isolate_fail;
>> +
>> + if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
>> + goto isolate_fail_put;
>> +
>> + /* Try isolate the page */
>> + if (!TestClearPageLRU(page))
>> + goto isolate_fail_put;
>> +
>> /* If we already hold the lock, we can skip some rechecking */
>> if (!locked) {
>> locked = compact_lock_irqsave(&pgdat->lru_lock,
>> @@ -980,10 +996,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> goto isolate_abort;
>> }
>>
>> - /* Recheck PageLRU and PageCompound under lock */
>> - if (!PageLRU(page))
>> - goto isolate_fail;
>> -
>> /*
>> * Page become compound since the non-locked check,
>> * and it's on LRU. It can only be a THP so the order
>> @@ -991,16 +1003,13 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> */
>> if (unlikely(PageCompound(page) && !cc->alloc_contig)) {
>> low_pfn += compound_nr(page) - 1;
>> - goto isolate_fail;
>> + SetPageLRU(page);
>> + goto isolate_fail_put;
>> }
>> }
>>
>> lruvec = mem_cgroup_page_lruvec(page, pgdat);
>>
>> - /* Try isolate the page */
>> - if (__isolate_lru_page(page, isolate_mode) != 0)
>> - goto isolate_fail;
>> -
>> /* The whole page is taken off the LRU; skip the tail pages. */
>> if (PageCompound(page))
>> low_pfn += compound_nr(page) - 1;
>> @@ -1029,6 +1038,15 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> }
>>
>> continue;
>> +
>> +isolate_fail_put:
>> + /* Avoid potential deadlock in freeing page under lru_lock */
>> + if (locked) {
>> + spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>> + locked = false;
>> + }
>> + put_page(page);
>> +
>> isolate_fail:
>> if (!skip_on_failure)
>> continue;
>> @@ -1065,9 +1083,15 @@ static bool too_many_isolated(pg_data_t *pgdat)
>> if (unlikely(low_pfn > end_pfn))
>> low_pfn = end_pfn;
>>
>> + page = NULL;
>> +
>> isolate_abort:
>> if (locked)
>> spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>> + if (page) {
>> + SetPageLRU(page);
>> + put_page(page);
>> + }
>>
>> /*
>> * Updated the cached scanner pfn once the pageblock has been scanned
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 1b3e0eeaad64..48b50695f883 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1538,20 +1538,20 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>> *
>> * returns 0 on success, -ve errno on failure.
>> */
>> -int __isolate_lru_page(struct page *page, isolate_mode_t mode)
>> +int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
>> {
>> int ret = -EINVAL;
>>
>> - /* Only take pages on the LRU. */
>> - if (!PageLRU(page))
>> - return ret;
>> -
>> /* Compaction should not handle unevictable pages but CMA can do so */
>> if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
>> return ret;
>>
>> ret = -EBUSY;
>>
>> + /* Only take pages on the LRU. */
>> + if (!PageLRU(page))
>> + return ret;
>> +
>
> So here you do deal with that BUG() issue. But I'd prefer you to leave
> it as I suggested in 16/32, just start with "int ret = -EBUSY;" and
> don't rearrange the checks here at all. I say that partly because
> the !PageLRU check is very important (when called for compaction), and
> the easier it is to find (at the very start), the less anxious I get!

yes, have done as your suggestion.

>
>> /*
>> * To minimise LRU disruption, the caller can indicate that it only
>> * wants to isolate pages it will be able to operate on without
>> @@ -1592,20 +1592,9 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
>> if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
>> return ret;
>>
>> - if (likely(get_page_unless_zero(page))) {
>> - /*
>> - * Be careful not to clear PageLRU until after we're
>> - * sure the page is not being freed elsewhere -- the
>> - * page release code relies on it.
>> - */
>> - ClearPageLRU(page);
>> - ret = 0;
>> - }
>> -
>> - return ret;
>> + return 0;
>> }
>>
>> -
>> /*
>> * Update LRU sizes after isolating pages. The LRU size updates must
>> * be complete before mem_cgroup_update_lru_size due to a sanity check.
>> @@ -1685,17 +1674,34 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>> * only when the page is being freed somewhere else.
>> */
>> scan += nr_pages;
>> - switch (__isolate_lru_page(page, mode)) {
>> + switch (__isolate_lru_page_prepare(page, mode)) {
>> case 0:
>> + /*
>> + * Be careful not to clear PageLRU until after we're
>> + * sure the page is not being freed elsewhere -- the
>> + * page release code relies on it.
>> + */
>> + if (unlikely(!get_page_unless_zero(page)))
>> + goto busy;
>> +
>> + if (!TestClearPageLRU(page)) {
>> + /*
>> + * This page may in other isolation path,
>> + * but we still hold lru_lock.
>> + */
>> + put_page(page);
>> + goto busy;
>> + }
>> +
>> nr_taken += nr_pages;
>> nr_zone_taken[page_zonenum(page)] += nr_pages;
>> list_move(&page->lru, dst);
>> break;
>> -
>> +busy:
>> case -EBUSY:
>
> It's a long time since I read a C manual. I had to try that out in a
> little test program: and it does seem to do the right thing. Maybe
> I'm just very ignorant, and everybody else finds that natural: but I'd
> feel more comfortable with the busy label on the line after the
> "case -EBUSY:" - wouldn't you?

will move down. Thanks!

>
> You could, of course, change that "case -EBUSY" to "default",
> and delete the "default: BUG();" that follows: whatever you prefer.
>

yes, the default is enough after last patch's change.

>> /* else it is being freed elsewhere */
>> list_move(&page->lru, src);
>> - continue;
>> + break;
>
> Aha. Yes, I like that change, I'm not going to throw a tantrum,
> accusing you of sneaking in unrelated changes etc. You made me look
> back at the history: it was "continue" from back in the days of
> lumpy reclaim, when there was stuff after the switch statement
> which needed to be skipped in the -EBUSY case. "break" looks
> more natural to me now.

Thanks!
with above 'default' change, the break could be saved finally. :)

Thanks!

>
>>
>> default:
>> BUG();
>> --
>> 1.8.3.1

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