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    SubjectRe: [PATCH] support O_DIRECT in tmpfs/ramfs


    Peter Staubach wrote:
    > Hua Zhong wrote:
    >> Hi,
    >>
    >> A while ago there was a discussion about supporting direct-io on tmpfs.
    >>
    >> Here is a simple patch that does it.
    >>
    >> 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.
    >>
    >> 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:
    >>
    >> 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?
    >
    > This would seem to create two different sets of O_DIRECT semantics,
    > wouldn't it? I think that it would be possible to develop an application
    > using one of these FS_RAM_BASED file systems as the testbed, but then be
    > surprised when the application failed to work on other file systems such
    > as ext3.

    As I'm ignorant with regard to what is needed for "compliant"
    support of O_DIRECT on tmpfs, what are the issues with actually implementing
    the proper semantics, including the alignment and any transfer length
    restrictions?

    My $.02 is that the implementation should be fully compliant with the
    current semantics or it shouldn't be implemented. And I think it should
    be implemented.

    Mike

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