linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Date:   Tue, 21 Dec 2021 13:16:23 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>
Cc:     David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Nadav Amit <namit@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

On 12/21/21 10:30, Linus Torvalds wrote:
> On Tue, Dec 21, 2021 at 10:07 AM Jan Kara <jack@...e.cz> wrote:
>>
>> For record we always intended (and still intend) to make O_DIRECT use
>> FOLL_PIN. Just it is tricky because some users mix pages pinned with GUP
>> and pages acquired through get_page() in a single bio (such as zero page)
>> and thus it is non-trivial to do the right thing on IO completion (unpin or
>> just put_page).
> 
> Side note: the new "exclusive VM" bit wouldn't _solve_ this issue, but
> it might make it much easier to debug and catch.
> 
> If we only set the exclusive VM bit on pages that get mapped into user
> space, and we guarantee that GUP only looks up such pages, then we can
> also add a debug test to the "unpin" case that the bit is still set.
> 
> And that would catch anybody who ends up using other pages for
> unpin(), and you could have a WARN_ON() for it (obviously also trigger
> on the page count being too small to unpin).
> 
> That way, at least from a kernel debugging and development standpoint
> it would make it easy to see "ok, this unpinning got a page that
> wasn't pinned", and it would help find these cases where some
> situation had used just a get_page() rather than a pin to get a page
> pointer.
> 
> No?
> 
>                    Linus

Yes, this is especially welcome, because it means that after enough time
sitting in the -mm tree, we can reasonably expect to catch the most important
cases, if any were missed. That makes it a whole other level of useful, as
compared to local testing hacks.


thanks,
-- 
John Hubbard
NVIDIA