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

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Message-ID: <ZMgCr4TxgNQZhhQK@x1n>
Date:   Mon, 31 Jul 2023 14:51:27 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        liubo <liubo254@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout

On Mon, Jul 31, 2023 at 11:23:59AM -0700, Linus Torvalds wrote:
> So GUP-fast can only look at the page table data, and as such *has* to
> fail if the page table is inaccessible.
> 
> But GUP in general? Why would it want to honor numa faulting?
> Particularly by default, and _particularly_ for things like
> FOLL_REMOTE.

True.

> 
> In fact, I feel like this is what the real rule should be: we simply
> define that get_user_pages_fast() is about looking up the page in the
> page tables.
> 
> So if you want something that acts like a page table lookup, you use
> that "fast" thing.  It's literally how it is designed. The whole - and
> pretty much only - point of it is that it can be used with no locking
> at all, because it basically acts like the hardware lookup does.

Unfortunately I think at least kvm (besides the rest..) relies not only on
numa balancing but also fast-gup.. :-( Please refer to hva_to_pfn() where
it even supports fast-gup-only HVA translation when atomic==true set.

Thanks,

-- 
Peter Xu