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  • Jens Axboe's avatar
    [PATCH] splice: page stealing needs to wait_on_page_writeback() · ad8d6f0a
    Jens Axboe authored
    Thanks to Andrew for the good explanation of why this is so. akpm writes:
    If a page is under writeback and we remove it from pagecache, it's still
    going to get written to disk.  But the VFS no longer knows about that page,
    nor that this page is about to modify disk blocks.
    So there might be scenarios in which those
    blocks-which-are-about-to-be-written-to get reused for something else.
    When writeback completes, it'll scribble on those blocks.
    This won't happen in ext2/ext3-style filesystems in normal mode because the
    page has buffers and try_to_release_page() will fail.
    But ext2 in nobh mode doesn't attach buffers at all - it just sticks the
    page in a BIO, finds some new blocks, points the BIO at those blocks and
    lets it rip.
    While that write IO's in flight, someone could truncate the file.  Truncate
    won't block on the writeout because the page isn't in pagecache any more.
    So truncate will the free the blocks from the file under the page's feet.
    Then something else can reallocate those blocks.  Then write data to them.
    Now, the original write completes, corrupting the filesystem.
    Signed-off-by: default avatarJens Axboe <>