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    SubjectRe: PATCH: killing read_ahead[]

    On Wed, 25 Oct 2000, Linus Torvalds wrote:

    > On Wed, 25 Oct 2000, Rik van Riel wrote:
    > >
    > > OTOH, block-dev readahead makes sense for filesystems where
    > > the packing locality is close to the access pattern BUT NOT
    > > close to anything the page cache would recognise as being
    > > close.
    > I dunno. The main reason I'd like to get the block devices into the page
    > cache is that right now there is no way to mmap them - something that can
    > potentially be _very_ useful, regardless of readahead.
    > And quite frankly, the generic file readahead has been pounded upon and
    > tested a lot more than the block device read-ahead ever was. I bet it
    > performs better if for no other reason.

    Umm... OK, I think that I can do it. Prerequisit: two new fields in
    address_space that make sense by themselves:
    (i_size + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT

    Can we add it now? I mean the following: we have _very_ few places where
    i_size is being changed. We could easily calculate these two expressions
    at such points and put them into ->i_mapping. That kills quite a few long
    long shifts (Good Thing, IMO) in places where we are using these values.

    _If_ we can have it on that stage - devices in pagecache are trivial.
    Frankly, I think that caching them makes sense regardless of
    device-related stuff.

    I can do the proof-of-concept patch (below 10Kb, ext2 + generic code, with
    the need to repeat the fs-specific parts for other filesystems) in an
    hour. Clearance?

    Notice that said fields do _not_ replace i_size. They mirror it and allow
    * avoid costly recaclulation
    * have generic_file_read / filemap_nopage free from references to
    inode->i_size, thus making them more usable for devices.

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