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    SubjectRe: [PATCH v6 0/7] vfs: Non-blockling buffered fs read (page cache only)
    On Tue, 2 Dec 2014 17:17:42 -0500 Milosz Tanski <milosz@adfin.com> wrote:

    > > There have been several incomplete attempts to implement fincore(). If
    > > we were to complete those attempts, preadv2() could be implemented
    > > using fincore()+pread(). Plus we get fincore(), which is useful for
    > > other (but probably similar) reasons. Probably fincore()+pwrite() could
    > > be used to implement pwritev2(), but I don't know what pwritev2() does
    > > yet.
    > >
    > > Implementing fincore() is more flexible, requires less code and is less
    > > likely to have bugs. So why not go that way? Yes, it's more CPU
    > > intensive, but how much? Is the difference sufficient to justify the
    > > preadv2()/pwritev2() approach?
    >
    > I would like to see a fincore() functionality (for other reasons) I
    > don't think it does the job here. fincore() + preadv() is inherently
    > racy as there's no guarantee that the data becomes uncached between
    > the two calls.

    There will always be holes. For example find_get_page() could block on
    lock_page() while some other process is doing IO.
    page_cache_async_readahead() does lots of memory allocation which can
    get blocked for long periods in the page allocator.
    page_cache_async_readahead() can block on synchronous metadata reads,
    etc.

    The question is whether a simpler approach such as fincore() will be
    sufficient.

    > This may not matter in some cases, but in others (ones
    > that I'm trying to solve) it will introduce unexpected latency.

    Details?

    > There's no overlap between prwritev2 and fincore() functionality.

    Do we actually need pwritev2()? What's the justification for that?


    Please let's examine the alternative(s) seriously. It would be mistake
    to add preadv2/pwritev2 if fincore+pread would have sufficed.



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