From: John Hubbard <jhubbard@nvidia.com>
To: David Hildenbrand <david@redhat.com>,
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 19:11:05 -0700 [thread overview]
Message-ID: <846b770c-9f63-90a2-0435-ec82484e3f74@nvidia.com> (raw)
In-Reply-To: <e31cd404-56ce-4cad-fcc3-3a6695f750fa@redhat.com>
On 5/26/23 11:50, David Hildenbrand wrote:
> 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.
Not yet, anyway. :)
>
>> 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().
>
I was actually thinking that we should minimize any more cases of
fragile mapcount and refcount comparison, which then leads to
Matthew's approach here!
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2023-05-27 2:11 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
2023-05-27 2:11 ` John Hubbard [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=846b770c-9f63-90a2-0435-ec82484e3f74@nvidia.com \
--to=jhubbard@nvidia.com \
--cc=akpm@linux-foundation.org \
--cc=baolin.wang@linux.alibaba.com \
--cc=david@redhat.com \
--cc=khalid.aziz@oracle.com \
--cc=khalid@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mgorman@techsingularity.net \
--cc=steven.sistare@oracle.com \
--cc=willy@infradead.org \
--cc=ying.huang@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).