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Date:   Wed, 22 Dec 2021 14:09:41 +0100
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Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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To:     Jan Kara <jack@suse.cz>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
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        Nadav Amit <namit@vmware.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20211222124141.GA685@quack2.suse.cz>
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>> 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