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    SubjectRe: [PATCH v11 2/9] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages
    Date
    On 10/21/22 10:41, Logan Gunthorpe wrote:
    > GUP Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
    > allow obtaining P2PDMA pages. If GUP is called without the flag and a
    > P2PDMA page is found, it will return an error in try_grab_page() or
    > try_grab_folio().
    >
    > The check is safe to do before taking the reference to the page in both
    > cases seeing the page should be protected by either the appropriate
    > ptl or mmap_lock; or the gup fast guarantees preventing TLB flushes.
    >
    > try_grab_folio() has one call site that WARNs on failure and cannot
    > actually deal with the failure of this function (it seems it will
    > get into an infinite loop). Expand the comment there to document a
    > couple more conditions on why it will not fail.
    >
    > FOLL_PCI_P2PDMA cannot be set if FOLL_LONGTERM is set. This is to copy
    > fsdax until pgmap refcounts are fixed (see the link below for more
    > information).
    >
    > Link: https://lkml.kernel.org/r/Yy4Ot5MoOhsgYLTQ@ziepe.ca
    > Signed-off-by: Logan Gunthorpe <logang@deltatee.com>

    Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>

    -ck
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