From: David Hildenbrand <david@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Khalid Aziz <khalid.aziz@oracle.com>,
Steven Sistare <steven.sistare@oracle.com>,
akpm@linux-foundation.org, ying.huang@intel.com,
mgorman@techsingularity.net, baolin.wang@linux.alibaba.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Khalid Aziz <khalid@kernel.org>
Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan
Date: Fri, 26 May 2023 20:50:19 +0200 [thread overview]
Message-ID: <e31cd404-56ce-4cad-fcc3-3a6695f750fa@redhat.com> (raw)
In-Reply-To: <ZHD+eOMpZpWXNAig@casper.infradead.org>
On 26.05.23 20:46, Matthew Wilcox wrote:
> On Fri, May 26, 2023 at 06:46:15PM +0200, David Hildenbrand wrote:
>> On 26.05.23 18:44, Matthew Wilcox wrote:
>>> On Fri, May 26, 2023 at 09:44:34AM -0600, Khalid Aziz wrote:
>>>>> Oh, I think I found it! pin_user_pages_remote() is called by
>>>>> vaddr_get_pfns(). If these are the pages you're concerned about,
>>>>> then the efficient way to do what you want is simply to call
>>>>> folio_maybe_dma_pinned(). Far more efficient than the current mess
>>>>> of total_mapcount().
>>>>
>>>> vfio pinned pages triggered this change. Wouldn't checking refcounts against
>>>> mapcount provide a more generalized way of detecting non-migratable pages?
>>>
>>> Well, you changed the comment to say that we were concerned about
>>> long-term pins. If we are, than folio_maybe_dma_pinned() is how to test
>>> for long-term pins. If we want to skip pages which are short-term pinned,
>>> then we need to not change the comment, and keep using mapcount/refcount
>>> differences.
>>>
>>
>> folio_maybe_dma_pinned() is all about FOLL_PIN, not FOLL_LONGTERM.
>
> But according to our documentation, FOLL_LONGTERM implies FOLL_PIN.
Yes. But folio_maybe_dma_pinned() will indicate both, long-term pins and
short-term pins. There really is no way to distinguish both, unfortunately.
> Anyway, right now, the code skips any pages which are merely FOLL_GET,
> so we'll skip fewer pages if we do only skip the FOLL_PIN ones,
> regardless if we'd prefer to only skip the FOLL_LONGTERM ones.
>
>> folio_maybe_dma_pinned() would skip migrating any page that has more than
>> 1024 references. (shared libraries?)
>
> True, but maybe we should be skipping any page with that many mappings,
> given how disruptive it is to the rest of the system to unmap a page
> from >1024 processes.
>
So any user with 1024 processes can fragment physical memory? :/
Sorry, I'd like to minimize the usage of folio_maybe_dma_pinned().
It's all suboptimal but let's not try to make it worse.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2023-05-26 18:51 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-25 19:15 [PATCH v4] mm, compaction: Skip all non-migratable pages during scan Khalid Aziz
2023-05-25 19:58 ` Matthew Wilcox
2023-05-25 20:15 ` Steven Sistare
2023-05-25 20:45 ` Matthew Wilcox
2023-05-25 21:31 ` Matthew Wilcox
2023-05-26 15:44 ` Khalid Aziz
2023-05-26 16:44 ` Matthew Wilcox
2023-05-26 16:46 ` David Hildenbrand
2023-05-26 18:46 ` Matthew Wilcox
2023-05-26 18:50 ` David Hildenbrand [this message]
2023-05-27 2:11 ` John Hubbard
2023-05-27 3:18 ` Matthew Wilcox
2023-05-28 23:49 ` John Hubbard
2023-05-29 0:31 ` Matthew Wilcox
2023-05-29 9:25 ` David Hildenbrand
2023-05-30 15:42 ` Khalid Aziz
2023-06-09 22:11 ` Andrew Morton
2023-06-09 23:28 ` Khalid Aziz
2023-05-25 20:41 ` Khalid Aziz
2023-05-29 3:01 ` Huang, Ying
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