linux-kernel - Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

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Message-ID: <140e7688-b66d-2f6d-fed8-e39da5045420@redhat.com>
Date:   Fri, 26 Aug 2022 18:46:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

On 26.08.22 17:55, Peter Xu wrote:
> On Fri, Aug 26, 2022 at 04:47:22PM +0200, David Hildenbrand wrote:
>>> To me anon exclusive only shows this mm exclusively owns this page. I
>>> didn't quickly figure out why that requires different handling on tlb
>>> flushs.  Did I perhaps miss something?
>>
>> GUP-fast is the magic bit, we have to make sure that we won't see new
>> GUP pins, thus the TLB flush.
>>
>> include/linux/mm.h:gup_must_unshare() contains documentation.
> 
> Hmm.. Shouldn't ptep_get_and_clear() (e.g., xchg() on x86_64) already
> guarantees that no other process/thread will see this pte anymore
> afterwards?

You could have a GUP-fast thread that just looked up the PTE and is
going to pin the page afterwards, after the ptep_get_and_clear()
returned. You'll have to wait until that thread finished.

Another user that relies on this interaction between GUP-fast and TLB
flushing is for example mm/ksm.c:write_protect_page()

There is a comment in there explaining the interaction a bit more detailed.

Maybe we'll be able to handle this differently in the future (maybe once
this turns out to be an actual performance problem). Unfortunately,
mm->write_protect_seq isn't easily usable because we'd need have to make
sure we're the exclusive writer.

For now, it's not too complicated. For PTEs:
* try_to_migrate_one() already uses ptep_clear_flush().
* try_to_unmap_one() already conditionally used ptep_clear_flush().
* migrate_vma_collect_pmd() was the one case that didn't use it already
 (and I wonder why it's different than try_to_migrate_one()).

-- 
Thanks,

David / dhildenb