linux-kernel - Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow

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Date:   Fri, 28 Jul 2023 19:54:54 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        liubo <liubo254@...wei.com>, Peter Xu <peterx@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout

[...]

> 
> There was the case for "FOLL_PIN represents application behavior and
> should trigger NUMA faults", but I guess that can be ignored.
> 
> But it would be much better to just remove all that if we can.
> 
> Let me look into some details.

I just stumbled over the comment from Mel in follow_trans_huge_pmd():

/* Full NUMA hinting faults to serialise migration in fault paths */

It dates back to

commit 2b4847e73004c10ae6666c2e27b5c5430aed8698
Author: Mel Gorman <mgorman@...e.de>
Date:   Wed Dec 18 17:08:32 2013 -0800

     mm: numa: serialise parallel get_user_page against THP migration
     
     Base pages are unmapped and flushed from cache and TLB during normal
     page migration and replaced with a migration entry that causes any
     parallel NUMA hinting fault or gup to block until migration completes.
     
     THP does not unmap pages due to a lack of support for migration entries
     at a PMD level.  This allows races with get_user_pages and
     get_user_pages_fast which commit 3f926ab945b6 ("mm: Close races between
     THP migration and PMD numa clearing") made worse by introducing a
     pmd_clear_flush().
     
     This patch forces get_user_page (fast and normal) on a pmd_numa page to
     go through the slow get_user_page path where it will serialise against
     THP migration and properly account for the NUMA hinting fault.  On the
     migration side the page table lock is taken for each PTE update.


We nowadays do have migration entries at PMD level -- and setting FOLL_FORCE
could similarly trigger such a race.

So I suspect we're good.

-- 
Cheers,

David / dhildenb