From: David Hildenbrand <david@redhat.com>
To: Ryan Roberts <ryan.roberts@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <willy@infradead.org>,
Huang Ying <ying.huang@intel.com>, Gao Xiang <xiang@kernel.org>,
Yu Zhao <yuzhao@google.com>, Yang Shi <shy828301@gmail.com>,
Michal Hocko <mhocko@suse.com>,
Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
Date: Mon, 4 Mar 2024 23:02:03 +0100 [thread overview]
Message-ID: <3d47ae7d-297a-441e-941c-5b2e34ba8759@redhat.com> (raw)
In-Reply-To: <6cfc022a-0c7a-4fe6-aaa4-3d28aeacc982@arm.com>
On 04.03.24 22:55, Ryan Roberts wrote:
> On 04/03/2024 20:50, David Hildenbrand wrote:
>>>>>
>>>>> This is the existing free_swap_and_cache(). I think _swap_info_get() would
>>>>> break
>>>>> if this could race with swapoff(), and __swap_entry_free() looks up the cluster
>>>>> from an array, which would also be freed by swapoff if racing:
>>>>>
>>>>> int free_swap_and_cache(swp_entry_t entry)
>>>>> {
>>>>> struct swap_info_struct *p;
>>>>> unsigned char count;
>>>>>
>>>>> if (non_swap_entry(entry))
>>>>> return 1;
>>>>>
>>>>> p = _swap_info_get(entry);
>>>>> if (p) {
>>>>> count = __swap_entry_free(p, entry);
>>>>
>>>> If count dropped to 0 and
>>>>
>>>>> if (count == SWAP_HAS_CACHE)
>>>>
>>>>
>>>> count is now SWAP_HAS_CACHE, there is in fact no swap entry anymore. We removed
>>>> it. That one would have to be reclaimed asynchronously.
>>>>
>>>> The existing code we would call swap_page_trans_huge_swapped() with the SI it
>>>> obtained via _swap_info_get().
>>>>
>>>> I also don't see what should be left protecting the SI. It's not locked anymore,
>>>> the swapcounts are at 0. We don't hold the folio lock.
>>>>
>>>> try_to_unuse() will stop as soon as si->inuse_pages is at 0. Hm ...
>>>
>>> But, assuming the caller of free_swap_and_cache() acquires the PTL first, I
>>> think this all works out ok? While free_swap_and_cache() is running,
>>> try_to_unuse() will wait for the PTL. Or if try_to_unuse() runs first, then
>>> free_swap_and_cache() will never be called because the swap entry will have been
>>> removed from the PTE?
>>
>> But can't try_to_unuse() run, detect !si->inuse_pages and not even bother about
>> scanning any further page tables?
>>
>> But my head hurts from digging through that code.
>
> Yep, glad I'm not the only one that gets headaches from swapfile.c.
>
>>
>> Let me try again:
>>
>> __swap_entry_free() might be the last user and result in "count == SWAP_HAS_CACHE".
>>
>> swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.
>>
>>
>> So the question is: could someone reclaim the folio and turn si->inuse_pages==0,
>> before we completed swap_page_trans_huge_swapped().
>>
>> Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are still
>> references by swap entries.
>>
>> Process 1 still references subpage 0 via swap entry.
>> Process 2 still references subpage 1 via swap entry.
>>
>> Process 1 quits. Calls free_swap_and_cache().
>> -> count == SWAP_HAS_CACHE
>> [then, preempted in the hypervisor etc.]
>>
>> Process 2 quits. Calls free_swap_and_cache().
>> -> count == SWAP_HAS_CACHE
>>
>> Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
>> __try_to_reclaim_swap().
>>
>> __try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->put_swap_folio()->
>> free_swap_slot()->swapcache_free_entries()->swap_entry_free()->swap_range_free()->
>> ...
>> WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);
>>
>>
>> What stops swapoff to succeed after process 2 reclaimed the swap cache but
>> before process 1 finished its call to swap_page_trans_huge_swapped()?
>
> Assuming you are talking about anonymous memory, process 1 has the PTL while
> it's executing free_swap_and_cache(). try_to_unuse() iterates over every vma in
> every mm, and it swaps-in a page for every PTE that holds a swap entry for the
> device being swapoff'ed. It takes the PTL while converting the swap entry to
> present PTE - see unuse_pte(). Process 1 must have beaten try_to_unuse() to the
> particular pte, because if try_to_unuse() got there first, it would have
> converted it from a swap entry to present pte and process 1 would never even
> have called free_swap_and_cache(). So try_to_unuse() will eventually wait on the
> PTL until process 1 has released it after free_swap_and_cache() completes. Am I
> missing something? Because that part feels pretty clear to me.
Why should try_to_unuse() do *anything* if it already finds
si->inuse_pages == 0 because we (p1 } p2) just freed the swapentries and
process 2 managed to free the last remaining swapcache entry?
I'm probably missing something important :)
try_to_unuse() really starts with
if (!READ_ONCE(si->inuse_pages))
goto success;
--
Cheers,
David / dhildenb
next prev parent reply other threads:[~2024-03-04 22:02 UTC|newest]
Thread overview: 116+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-25 14:45 [PATCH v3 0/4] Swap-out small-sized THP without splitting Ryan Roberts
2023-10-25 14:45 ` [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags Ryan Roberts
2024-02-22 10:19 ` David Hildenbrand
2024-02-22 10:20 ` David Hildenbrand
2024-02-26 17:41 ` Ryan Roberts
2024-02-27 17:10 ` Ryan Roberts
2024-02-27 19:17 ` David Hildenbrand
2024-02-28 9:37 ` Ryan Roberts
2024-02-28 12:12 ` David Hildenbrand
2024-02-28 14:57 ` Ryan Roberts
2024-02-28 15:12 ` David Hildenbrand
2024-02-28 15:18 ` Ryan Roberts
2024-03-01 16:27 ` Ryan Roberts
2024-03-01 16:31 ` Matthew Wilcox
2024-03-01 16:44 ` Ryan Roberts
2024-03-01 17:00 ` David Hildenbrand
2024-03-01 17:14 ` Ryan Roberts
2024-03-01 17:18 ` David Hildenbrand
2024-03-01 17:06 ` Ryan Roberts
2024-03-04 4:52 ` Barry Song
2024-03-04 5:42 ` Barry Song
2024-03-05 7:41 ` Ryan Roberts
2024-03-01 16:31 ` Ryan Roberts
2024-03-01 16:32 ` David Hildenbrand
2024-03-04 16:03 ` Ryan Roberts
2024-03-04 17:30 ` David Hildenbrand
2024-03-04 18:38 ` Ryan Roberts
2024-03-04 20:50 ` David Hildenbrand
2024-03-04 21:55 ` Ryan Roberts
2024-03-04 22:02 ` David Hildenbrand [this message]
2024-03-04 22:34 ` Ryan Roberts
2024-03-05 6:11 ` Huang, Ying
2024-03-05 8:35 ` David Hildenbrand
2024-03-05 8:46 ` Ryan Roberts
2024-02-28 13:33 ` Matthew Wilcox
2024-02-28 14:24 ` Ryan Roberts
2024-02-28 14:59 ` Ryan Roberts
2023-10-25 14:45 ` [PATCH v3 2/4] mm: swap: Remove struct percpu_cluster Ryan Roberts
2023-10-25 14:45 ` [PATCH v3 3/4] mm: swap: Simplify ssd behavior when scanner steals entry Ryan Roberts
2023-10-25 14:45 ` [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting Ryan Roberts
2023-10-30 8:18 ` Huang, Ying
2023-10-30 13:59 ` Ryan Roberts
2023-10-31 8:12 ` Huang, Ying
2023-11-03 11:42 ` Ryan Roberts
2023-11-02 7:40 ` Barry Song
2023-11-02 10:21 ` Ryan Roberts
2023-11-02 22:36 ` Barry Song
2023-11-03 11:31 ` Ryan Roberts
2023-11-03 13:57 ` Steven Price
2023-11-04 9:34 ` Barry Song
2023-11-06 10:12 ` Steven Price
2023-11-06 21:39 ` Barry Song
2023-11-08 11:51 ` Steven Price
2023-11-07 12:46 ` Ryan Roberts
2023-11-07 18:05 ` Barry Song
2023-11-08 11:23 ` Barry Song
2023-11-08 20:20 ` Ryan Roberts
2023-11-08 21:04 ` Barry Song
2023-11-04 5:49 ` Barry Song
2024-02-05 9:51 ` Barry Song
2024-02-05 12:14 ` Ryan Roberts
2024-02-18 23:40 ` Barry Song
2024-02-20 20:03 ` Ryan Roberts
2024-03-05 9:00 ` Ryan Roberts
2024-03-05 9:54 ` Barry Song
2024-03-05 10:44 ` Ryan Roberts
2024-02-27 12:28 ` Ryan Roberts
2024-02-27 13:37 ` Ryan Roberts
2024-02-28 2:46 ` Barry Song
2024-02-22 7:05 ` Barry Song
2024-02-22 10:09 ` David Hildenbrand
2024-02-23 9:46 ` Barry Song
2024-02-27 12:05 ` Ryan Roberts
2024-02-28 1:23 ` Barry Song
2024-02-28 9:34 ` David Hildenbrand
2024-02-28 23:18 ` Barry Song
2024-02-28 15:57 ` Ryan Roberts
2023-11-29 7:47 ` [PATCH v3 0/4] " Barry Song
2023-11-29 12:06 ` Ryan Roberts
2023-11-29 20:38 ` Barry Song
2024-01-18 11:10 ` [PATCH RFC 0/6] mm: support large folios swap-in Barry Song
2024-01-18 11:10 ` [PATCH RFC 1/6] arm64: mm: swap: support THP_SWAP on hardware with MTE Barry Song
2024-01-26 23:14 ` Chris Li
2024-02-26 2:59 ` Barry Song
2024-01-18 11:10 ` [PATCH RFC 2/6] mm: swap: introduce swap_nr_free() for batched swap_free() Barry Song
2024-01-26 23:17 ` Chris Li
2024-02-26 4:47 ` Barry Song
2024-01-18 11:10 ` [PATCH RFC 3/6] mm: swap: make should_try_to_free_swap() support large-folio Barry Song
2024-01-26 23:22 ` Chris Li
2024-01-18 11:10 ` [PATCH RFC 4/6] mm: support large folios swapin as a whole Barry Song
2024-01-27 19:53 ` Chris Li
2024-02-26 7:29 ` Barry Song
2024-01-27 20:06 ` Chris Li
2024-02-26 7:31 ` Barry Song
2024-01-18 11:10 ` [PATCH RFC 5/6] mm: rmap: weaken the WARN_ON in __folio_add_anon_rmap() Barry Song
2024-01-18 11:54 ` David Hildenbrand
2024-01-23 6:49 ` Barry Song
2024-01-29 3:25 ` Chris Li
2024-01-29 10:06 ` David Hildenbrand
2024-01-29 16:31 ` Chris Li
2024-02-26 5:05 ` Barry Song
2024-04-06 23:27 ` Barry Song
2024-01-27 23:41 ` Chris Li
2024-01-18 11:10 ` [PATCH RFC 6/6] mm: madvise: don't split mTHP for MADV_PAGEOUT Barry Song
2024-01-29 2:15 ` Chris Li
2024-02-26 6:39 ` Barry Song
2024-02-27 12:22 ` Ryan Roberts
2024-02-27 22:39 ` Barry Song
2024-02-27 14:40 ` Ryan Roberts
2024-02-27 18:57 ` Barry Song
2024-02-28 3:49 ` Barry Song
2024-01-18 15:25 ` [PATCH RFC 0/6] mm: support large folios swap-in Ryan Roberts
2024-01-18 23:54 ` Barry Song
2024-01-19 13:25 ` Ryan Roberts
2024-01-27 14:27 ` Barry Song
2024-01-29 9:05 ` Huang, Ying
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