linux-kernel - Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan

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Message-ID: <e31cd404-56ce-4cad-fcc3-3a6695f750fa@redhat.com>
Date:   Fri, 26 May 2023 20:50:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        akpm@...ux-foundation.org, ying.huang@...el.com,
        mgorman@...hsingularity.net, baolin.wang@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Khalid Aziz <khalid@...nel.org>
Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during
 scan

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