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    SubjectRe: [PATCH] VFS readahead bug in 2.6.8-rc[1-3]
    Nick Piggin wrote:

    > Phillip Lougher wrote:
    >> Hi,
    >> There is a readahead bug in do_generic_mapping_read (filemap.c). This
    >> bug appears to have been introduced in 2.6.8-rc1. Specifically the bug
    >> is caused by an incorrect code change which causes VFS to call
    >> readpage() for indexes beyond the end of files where the file length is
    >> zero or a 4k multiple.
    >> In Squashfs this causes a variety of almost immediate OOPes because
    >> Squashfs trusts the VFS not to pass invalid index values. For other
    >> filesystems it may also be causing subtle bugs. I have received
    >> prune_dcache oopes similar to Gene Heskett's (which was also
    >> pointer corruption), and so it may fix this and other reported
    >> readahead bugs.
    >> The patch is against 2.6.8-rc3.
    > Good work - bug is mine, sorry.

    On second thought, maybe not. I think your filesystem is at fault.

    Firstly, there possibly is a bug in do_pagecache_readahead which allows
    it to
    read off the end of the file in a completely serialised situation. But
    even so,
    notice the absence of locking - i_size can change at any time can't it? Then
    your fix will blow up when you race with a truncate, right?

    I think you need to handle reading off the end of the file properly: I'm not
    too familiar with this code, but IIRC you are allowed to setup pagecache
    the end of the file - it gets handled correctly in
    do_generic_mapping_read, and
    ends up falling off the LRU. This should help you.

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