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    SubjectRe: [PATCH v1 1/1] fs/splice: add missing callback for inaccessible pages
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    On 30.04.20 21:02, Christian Borntraeger wrote:
    > On 30.04.20 20:12, Christian Borntraeger wrote:
    >>
    >>
    >> On 29.04.20 18:07, Dave Hansen wrote:
    >>> On 4/28/20 3:50 PM, Claudio Imbrenda wrote:
    >>>> If a page is inaccesible and it is used for things like sendfile, then
    >>>> the content of the page is not always touched, and can be passed
    >>>> directly to a driver, causing issues.
    >>>>
    >>>> This patch fixes the issue by adding a call to arch_make_page_accessible
    >>>> in page_cache_pipe_buf_confirm; this fixes the issue.
    >>>
    >>> I spent about 5 minutes putting together a patch:
    >>>
    >>> https://sr71.net/~dave/intel/accessible.patch
    >>
    >> You only set the page flag for compound pages. that of course leaves a big pile
    >> of pages marked a not accessible, thus explaining the sendto trace and all kind
    >> of other random traces.
    >>
    >>
    >> What do you see when you also do the SetPageAccessible(page);
    >> in the else page of prep_new_page (order == 0).
    >> (I do get > 10000 of these non compound page allocs just during boot).
    >>
    >
    > And yes, I think you are right that we should call the callback also for !FOLL_PIN.


    Thinking again about this I am no longer sure. Adding John Hubbard.

    Documentation/core-api/pin_user_pages.rst says:
    -------snip----------
    Another way of thinking about these flags is as a progression of restrictions:
    FOLL_GET is for struct page manipulation, without affecting the data that the
    struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for
    short term pins on pages whose data *will* get accessed. As such, FOLL_PIN is
    a "more severe" form of pinning. And finally, FOLL_LONGTERM is an even more
    restrictive case that has FOLL_PIN as a prerequisite: this is for pages that
    will be pinned longterm, and whose data will be accessed.
    -------snip----------

    So John,is it ok to give a page to an I/O device where the code has used gup
    with FOLL_GET (or gup fast without pup) or would you consider this a bug?

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