From: Ryan Roberts <ryan.roberts@arm.com>
To: Barry Song <21cnbao@gmail.com>
Cc: akpm@linux-foundation.org, david@redhat.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com,
willy@infradead.org, xiang@kernel.org, ying.huang@intel.com,
yuzhao@google.com, chrisl@kernel.org, surenb@google.com,
hanchuanhua@oppo.com
Subject: Re: [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting
Date: Tue, 5 Mar 2024 09:00:54 +0000 [thread overview]
Message-ID: <7061b9f4-b7aa-4dad-858c-53ee186c2d8f@arm.com> (raw)
In-Reply-To: <CAGsJ_4wo7BiJWSKb1K_WyAai30KmfckMQ3-mCJPXZ892CtXpyQ@mail.gmail.com>
Hi Barry,
On 18/02/2024 23:40, Barry Song wrote:
> On Tue, Feb 6, 2024 at 1:14 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 05/02/2024 09:51, Barry Song wrote:
>>> +Chris, Suren and Chuanhua
>>>
>>> Hi Ryan,
[...]
>>
>
> Hi Ryan,
> I am running into some races especially while enabling large folio swap-out and
> swap-in both. some of them, i am still struggling with the detailed
> timing how they
> are happening.
> but the below change can help remove those bugs which cause corrupted data.
I'm getting quite confused with all the emails flying around on this topic. Here
you were reporting a data corruption bug and your suggested fix below is the one
you have now posted at [1]. But in the thread at [1] we concluded that it is not
fixing a functional correctness issue, but is just an optimization in some
corner cases. So does the corruption issue still manifest? Did you manage to
root cause it? Is it a problem with my swap-out series or your swap-in series,
or pre-existing?
[1] https://lore.kernel.org/linux-mm/20240304103757.235352-1-21cnbao@gmail.com/
Thanks,
Ryan
>
> index da2aab219c40..ef9cfbc84760 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1953,6 +1953,16 @@ static unsigned int shrink_folio_list(struct
> list_head *folio_list,
>
> if (folio_test_pmd_mappable(folio))
> flags |= TTU_SPLIT_HUGE_PMD;
> + /*
> + * make try_to_unmap_one hold ptl from the very first
> + * beginning if we are reclaiming a folio with multi-
> + * ptes. otherwise, we may only reclaim a part of the
> + * folio from the middle.
> + * for example, a parallel thread might temporarily
> + * set pte to none for various purposes.
> + */
> + else if (folio_test_large(folio))
> + flags |= TTU_SYNC;
>
> try_to_unmap(folio, flags);
> if (folio_mapped(folio)) {
>
>
> While we are swapping-out a large folio, it has many ptes, we change those ptes
> to swap entries in try_to_unmap_one(). "while (page_vma_mapped_walk(&pvmw))"
> will iterate all ptes within the large folio. but it will only begin
> to acquire ptl when
> it meets a valid pte as below /* xxxxxxx */
>
> static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> {
> pte_t ptent;
>
> if (pvmw->flags & PVMW_SYNC) {
> /* Use the stricter lookup */
> pvmw->pte = pte_offset_map_lock(pvmw->vma->vm_mm, pvmw->pmd,
> pvmw->address, &pvmw->ptl);
> *ptlp = pvmw->ptl;
> return !!pvmw->pte;
> }
>
> ...
> pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> pvmw->address, ptlp);
> if (!pvmw->pte)
> return false;
>
> ptent = ptep_get(pvmw->pte);
>
> if (pvmw->flags & PVMW_MIGRATION) {
> if (!is_swap_pte(ptent))
> return false;
> } else if (is_swap_pte(ptent)) {
> swp_entry_t entry;
> ...
> entry = pte_to_swp_entry(ptent);
> if (!is_device_private_entry(entry) &&
> !is_device_exclusive_entry(entry))
> return false;
> } else if (!pte_present(ptent)) {
> return false;
> }
> pvmw->ptl = *ptlp;
> spin_lock(pvmw->ptl); /* xxxxxxx */
> return true;
> }
>
>
> for various reasons, for example, break-before-make for clearing access flags
> etc. pte can be set to none. since page_vma_mapped_walk() doesn't hold ptl
> from the beginning, it might only begin to set swap entries from the middle of
> a large folio.
>
> For example, in case a large folio has 16 ptes, and 0,1,2 are somehow zero
> in the intermediate stage of a break-before-make, ptl will be held
> from the 3rd pte,
> and swap entries will be set from 3rd pte as well. it seems not good as we are
> trying to swap out a large folio, but we are swapping out a part of them.
>
> I am still struggling with all the timing of races, but using PVMW_SYNC to
> explicitly ask for ptl from the first pte seems a good thing for large folio
> regardless of those races. it can avoid try_to_unmap_one reading intermediate
> pte and further make the wrong decision since reclaiming pte-mapped large
> folios is atomic with just one pte.
>
>> Sorry I haven't progressed this series as I had hoped. I've been concentrating
>> on getting the contpte series upstream. I'm hoping I will find some time to move
>> this series along by the tail end of Feb (hoping to get it in shape for v6.10).
>> Hopefully that doesn't cause you any big problems?
>
> no worries. Anyway, we are already using your code to run various tests.
>
>>
>> Thanks,
>> Ryan
>
> Thanks
> Barry
next prev parent reply other threads:[~2024-03-05 9:01 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
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 [this message]
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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