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    SubjectRe: [PATCH] support O_DIRECT in tmpfs/ramfs
    Hi Hua Zhong,

    Maybe I misunderstand your patch, but when I tried it on my blackfin
    uClinux platform, I can't change anything. See below:
    root:/var> mount
    /dev/mtdblock0 on / type ext2 (rw)
    /proc on /proc type proc (rw)
    ramfs on /var type ramfs (rw)
    sysfs on /sys type sysfs (rw)
    devpts on /dev/pts type devpts (rw)
    root:/var> ./t_direct
    Error open t.bin to read
    Error is because O_DIRECT flag was set when call open(). If I remove this flag,
    the test program can work ok.

    Any suggestions?


    On 1/10/07, Hua Zhong <> wrote:
    > > > Here is a simple patch that does it.
    > >
    > > Looks more likely to work than Ken's - which I didn't try,
    > > but I couldn't see what magic prevented it from just going BUG.
    > >
    > > But I have to say, having seen the ensuing requests for this
    > > to impose the same constraints as other implementations of
    > > O_DIRECT (though NFS does not), I've veered right back to my
    > > original position: this all just seems silly to me. O_DIRECT
    > > is and always has been rather an awkward hack (Linus
    > > described it in stronger terms!), supported by many but not
    > > all filesystems: shall we just leave it at that?
    > So I take your word that NFS does not impose this restraint,
    > which means filesystems could choose their own alignment
    > requirement that makes sense. So it would not be too horrible
    > if tmpfs chooses to be liberal.
    > In fact, in the O_DIRECT man page it says:
    > O_DIRECT
    > [....] Under Linux 2.4 transfer sizes, and the alignment of
    > user buffer and file offset must all be multiples of the logi-
    > cal block size of the file system. Under Linux 2.6 alignment to
    > 512-byte boundaries suffices.
    > So even Linux 2.4 and 2.6 are different - 2.6 is less restrictive.
    > My point is that as long as we don't put more restrictions, it should
    > not cause real problems.
    > And about Linus..let's put his comment aside because O_DIRECT
    > is there to stay. :-) In fact, since O_DIRECT is not the most
    > beaufitul piece of code in the kernel, what I try to do is just to
    > make software developer's life easier by making filesystem
    > behavior as close to each other as possible with the minimal
    > effort.
    > > In particular, having now looked into the code, I'm amused to
    > > be reminded that one of its particular effects is to
    > > invalidate the pagecache for the area directly written.
    > > Well, it's hardly going to be worth replicating that
    > > behaviour with tmpfs or ramfs; yet if we don't, then we stand
    > > accused of it behaving misleadingly differently on them.
    > >
    > > I think Michael, who started off this discussion, did just the right
    > > thing: used a direct_IO filesystem on a loop device on a tmpfs file.
    > That's a rather heavy-weight workaround don't you think?
    > > > 1. A new fs flag FS_RAM_BASED is added and the O_DIRECT
    > > flag is ignored
    > > > if this flag is set (suggestions on a better name?)
    > > >
    > > > 2. Specify FS_RAM_BASED for tmpfs and ramfs.
    > >
    > > If this is pursued (not my preference, but let me stand aside
    > > now), you'd want to add in at least hugetlbfs and
    > > tiny-shmem.c. And set your (renamed) FS_RAM_BASED flag in
    > > ext2_aops_xip: that seems to be what they're wanting, then
    > > you can remove that strange test for
    > > f->f_mapping->a_ops->get_xip_page from __dentry_open.
    > >
    > > >
    > > > 3. When EINVAL is returned only a fput is done. I changed it to go
    > > > through cleanup_all. But there is still a cleanup problem:
    > >
    > > Is that change correct? Are you saying that the existing
    > > code leaks some structures? If so, please do send a patch to
    > > fix just that as soon as you can. But are you sure?
    > Having looked at the code more closely, the change is probably
    > not correct. fput(f) apparently does everything cleanup_all does,
    > and more, despite it's a single call. I guess those names are
    > a bit confusing at first glance. :-)
    > > > If a new file is created and then EINVAL is returned due to
    > > > O_DIRECT, the file is still left on the disk. I am not exactly
    > > > sure how to fix it other than adding another fs flag so we
    > > > could check O_DIRECT support at a much earlier stage.
    > > > Comments on how to fix it?
    > >
    > > None from me, sorry. It's untidy, but not a new issue you
    > > have to fix.
    > Well, looks like people are not in consensus to add the tmpfs
    > direct-io support, but since I've looked at the code, it would be
    > nice to fix this bug though.
    > The get_xip_page thing you mentioned makes it a bit more
    > complicated since XIP support is a mount option, not a
    > register_filesystem time option. If we ought to add a flag somewhere,
    > where is the right place? vfsmount?
    > I can cook up a patch for this bug if people think it's worth fixing.
    > -
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