From: Miaohe Lin <linmiaohe@huawei.com>
To: "ying.huang@intel.com" <ying.huang@intel.com>,
David Hildenbrand <david@redhat.com>,
Oscar Salvador <osalvador@suse.de>
Cc: <akpm@linux-foundation.org>, <songmuchun@bytedance.com>,
<hch@infradead.org>, <willy@infradead.org>, <linux-mm@kvack.org>,
<linux-kernel@vger.kernel.org>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
John Hubbard <jhubbard@nvidia.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>, Yu Zhao <yuzhao@google.com>
Subject: Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page check
Date: Sat, 16 Apr 2022 10:34:02 +0800 [thread overview]
Message-ID: <70b64476-6efc-c8ad-9cf3-b101c3b92db1@huawei.com> (raw)
In-Reply-To: <4b47e6317aca3deeabf610a7f4839563ff2b25a1.camel@intel.com>
On 2022/4/15 11:07, ying.huang@intel.com wrote:
> On Wed, 2022-04-13 at 09:26 +0800, ying.huang@intel.com wrote:
>> On Tue, 2022-04-12 at 16:59 +0200, David Hildenbrand wrote:
>>> On 12.04.22 15:42, Miaohe Lin wrote:
>>>> On 2022/4/12 16:59, Oscar Salvador wrote:
>>>>> On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote:
>>>>>> We don't need to check can_split_folio() because folio_maybe_dma_pinned()
>>>>>> is checked before. It will avoid the long term pinned pages to be swapped
>>>>>> out. And we can live with short term pinned pages. Without can_split_folio
>>>>>> checking we can simplify the code. Also activate_locked can be changed to
>>>>>> keep_locked as it's just short term pinning.
>>>>>
>>>>> What do you mean by "we can live with short term pinned pages"?
>>>>> Does it mean that it was not pinned when we check
>>>>> folio_maybe_dma_pinned() but now it is?
>>>>>
>>>>> To me it looks like the pinning is fluctuating and we rely on
>>>>> split_folio_to_list() to see whether we succeed or not, and if not
>>>>> we give it another spin in the next round?
>>>>
>>>> Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be
>>>> pinned for a noticeable time. So it's expected to split the folio successfully in the next
>>>> round as the pinning is really fluctuating. Or am I miss something?
>>>>
>>>
>>> Just so we're on the same page. folio_maybe_dma_pinned() only capture
>>> FOLL_PIN, but not FOLL_GET. You can have long-term FOLL_GET right now
>>> via vmsplice().
>>
>> Per my original understanding, folio_maybe_dma_pinned() can be used to
>> detect long-term pinned pages. And it seems reasonable to skip the
>> long-term pinned pages and try short-term pinned pages during page
>> reclaiming. But as you pointed out, vmsplice() doesn't use FOLL_PIN.
>> So if vmsplice() is expected to pin pages for long time, and we have no
>> way to detect it, then we should keep can_split_folio() in the original
>> code.
>>
>> Copying more people who have worked on long-term pinning for comments.
>
> Checked the discussion in the following thread,
>
> https://lore.kernel.org/lkml/CA+CK2bBffHBxjmb9jmSKacm0fJMinyt3Nhk8Nx6iudcQSj80_w@mail.gmail.com/
>
> It seems that from the practical point of view, folio_maybe_dma_pinned()
> can identify most long-term pinned pages that may block memory hot-
> remove or CMA allocation. Although as David pointed out, some pages may
> still be GUPed for long time (e.g. via vmsplice) even if
> !folio_maybe_dma_pinned().
>
> But from another point of view, can_split_huge_page() is cheap and THP
> swapout is expensive (swap space, disk IO, and hard to be recovered), so
> it may be better to keep can_split_huge_page() in shink_page_list().
Many thanks for your explanation. Looks convincing for me. Is it worth a comment about the above
stuff? Anyway, will drop this patch. Thanks!
>
> Best Regards,
> Huang, Ying
>
>>
>>> can_split_folio() is more precise then folio_maybe_dma_pinned(), but
>>> both are racy as long as the page is still mapped.
>>>
>>>
>>
>
>
> .
>
next prev parent reply other threads:[~2022-04-16 2:34 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-09 9:34 [PATCH v2 0/9] A few cleanup and fixup patches for vmscan Miaohe Lin
2022-04-09 9:34 ` [PATCH v2 1/9] mm/vmscan: add a comment about MADV_FREE pages check in folio_check_dirty_writeback Miaohe Lin
2022-04-11 11:50 ` ying.huang
2022-04-12 3:07 ` Miaohe Lin
2022-04-11 14:16 ` Christoph Hellwig
2022-04-12 3:15 ` Miaohe Lin
2022-04-12 8:44 ` Oscar Salvador
2022-04-09 9:34 ` [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page check Miaohe Lin
2022-04-11 14:18 ` Christoph Hellwig
2022-04-12 3:14 ` Miaohe Lin
2022-04-12 0:52 ` ying.huang
2022-04-12 8:59 ` Oscar Salvador
2022-04-12 13:42 ` Miaohe Lin
2022-04-12 14:59 ` David Hildenbrand
2022-04-13 1:26 ` ying.huang
2022-04-13 2:17 ` Miaohe Lin
2022-04-15 3:07 ` ying.huang
2022-04-16 2:34 ` Miaohe Lin [this message]
2022-04-09 9:34 ` [PATCH v2 3/9] mm/vmscan: introduce helper function reclaim_page_list() Miaohe Lin
2022-04-09 13:44 ` Matthew Wilcox
2022-04-11 1:53 ` Miaohe Lin
2022-04-11 3:17 ` Matthew Wilcox
2022-04-11 3:26 ` Miaohe Lin
2022-04-09 9:34 ` [PATCH v2 4/9] mm/vmscan: save a bit of stack space in shrink_lruvec Miaohe Lin
2022-04-12 0:57 ` ying.huang
2022-04-12 3:13 ` Miaohe Lin
2022-04-09 9:34 ` [PATCH v2 5/9] mm/vmscan: activate swap-backed executable folios after first usage Miaohe Lin
2022-04-12 0:59 ` ying.huang
2022-04-09 9:34 ` [PATCH v2 6/9] mm/vmscan: take all base pages of THP into account when race with speculative reference Miaohe Lin
2022-04-09 9:34 ` [PATCH v2 7/9] mm/vmscan: take min_slab_pages into account when try to call shrink_node Miaohe Lin
2022-04-12 1:36 ` ying.huang
2022-04-09 9:34 ` [PATCH v2 8/9] mm/vmscan: remove obsolete comment in kswapd_run Miaohe Lin
2022-04-09 9:35 ` [PATCH v2 9/9] mm/vmscan: use helper folio_is_file_lru() Miaohe Lin
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