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    SubjectRe: Q about pagecache data never written to disk
    Andrey Savochkin <saw@saw.sw.com.sg> wrote:
    >
    > Let's suppose an mmap'ed (SHARED, RW) file has a hole.
    > AFAICS, we allow to dirty the file pages without allocating the space for the
    > hole - filemap_nopage just "reads" the page filling it with zeroes, and
    > nothing is done about the on-disk data until writepage.
    >
    > So, if the page can't be written to disk (no space), the dirty data just
    > stays in the pagecache. The data can be read or seen via mmap, but it isn't
    > and never be on disk. The pagecache stays unsynchronized with the on-disk
    > content forever.

    The kernel will make one attampt to write the data to disk. If that write
    hits ENOSPC, the page is not redirtied (ie: the data can be lost).

    When that write hits ENOSPC an error flag is set in the address_space and
    that will be returned from a subsequent msync(). The application will then
    need to do something about it.

    If your application doesn't msync() the memory then it doesn't care about
    its data anyway. If your application _does_ msync the pages then we
    reliably report errors.

    > Is it the intended behavior?
    > Shouldn't we call the filesystem to fill the hole at the moment of the first
    > write access?

    That would be a retrograde step - it would be nice to move in the other
    direction: perform disk allocation at writeback time rather than at write()
    time, even for regular write() data. To do that we (probably) need space
    reservation APIs. And yes, we perhaps could reserve space in the
    filesystem when that page is first written to.

    But then what would we do if there's no space? SIGBUS? SIGSEGV?
    Inappropriate. SIGENOSPC?

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