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    SubjectRe: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)
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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

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