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

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Message-ID: <4a28e8a0-2efa-8b5e-10b5-38f1fc143a98@redhat.com>
Date:   Wed, 22 Dec 2021 14:09:41 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        John Hubbard <jhubbard@...dia.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)

>> IIUC, our COW logic makes sure that a shared anonymous page that might
>> still be used by a R/O FOLL_GET cannot be modified, because any attempt
>> to modify it would result in a copy.
> 
> Well, we defined FOLL_PIN to mean the intent that the caller wants to access
> not only page state (for which is enough FOLL_GET and there are some users
> - mostly inside mm - who need this) but also page data. Eventually, we even
> wanted to make FOLL_GET unavailable to broad areas of kernel (and keep it
> internal to only MM for its dirty deeds ;)) to reduce the misuse of GUP.
> 
> For file pages we need this data vs no-data access distinction so that
> filesystems can detect when someone can be accessing page data although the
> page is unmapped.  Practically, filesystems care most about when someone
> can be *modifying* page data (we need to make sure data is stable e.g. when
> writing back data to disk or doing data checksumming or other operations)
> so using FOLL_GET when wanting to only read page data should be OK for
> filesystems but honestly I would be reluctant to break the rule of "use
> FOLL_PIN when wanting to access page data" to keep things simple and
> reasonably easy to understand for parties such as filesystem developers or
> driver developers who all need to interact with pinned pages...

Right, from an API perspective we really want people to use FOLL_PIN.

To optimize this case in particular it would help if we would have the
FOLL flags on the unpin path. Then we could just decide internally
"well, short-term R/O FOLL_PIN can be really lightweight, we can treat
this like a FOLL_GET instead". And we would need that as well if we were
to keep different counters for R/O vs. R/W pinned.

-- 
Thanks,

David / dhildenb