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    SubjectRe: block device/VM question
    "A month of sundays ago Thunder from the hill wrote:"
    > On Tue, 27 Aug 2002, Peter T. Breuer wrote:
    > > Yes, I've noticed this in the 2.5.31 kernel. This is something
    > > to be done on open by the overlying fs or userspace utility, or
    > > should I set the flag on the inode->filp (or whatever) myself in
    > > the drivers open function? And do I need to define
    > > inode->mapping->a_ops->direct_IO()? (sorry - but I haven't had time to
    > > experiment yet today!).
    > Why so rough?

    Rough? You mean "approximate"? I was working from my vague mamory of
    what I read in the code on the train this morning.

    > > It looks like the 2.5 kernel has this pathway, but what about the 2.4
    > > kernel? Nothing.
    > The manpage claims:
    > O_DIRECT
    > This flag is supported on a number of Unix-like systems;
    > support was added under Linux in kernel version 2.4.10.

    My manpage for open(2) claims no such thing. It doesn't mention
    O_DIRECT. Maybe your libc6 is newer.

    In any case, it doesn't make it clear to me if I have to set the flag
    in the driver. Well, anyway, I'll set it.

    > Also:
    > A semantically similar interface for block devices is
    > described in raw(8).

    That's nothing like, since it's a character device that they were
    describing. But yes, reading drivers/char/raw.c led me to see the
    pathway via direct_IO() in 2.5.31, but in the 2.4.19 kernel the path is
    obscure. In 2.5.31 it's clear that mm/filemap.c does pay attention to the
    flag, and YES, you are right, in 2.4.19, generic_file_read() does
    look at the flag. However, I have not much idea where that filp
    comes from or what it represents. Oh, well, thanks.I suppose
    you won't give me a recipe saying "do this and your device won't be
    cached", but I'll follow the lead.

    Thanks again.


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