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ext4: Fix mmap/truncate race when blocksize < pagesize && delayed allocation
It is possible to see buffer_heads which are not mapped in the writepage callback in the following scneario (where the fs blocksize is 1k and the page size is 4k): 1) truncate(f, 1024) 2) mmap(f, 0, 4096) 3) a[0] = 'a' 4) truncate(f, 4096) 5) writepage(...) Now if we get a writepage callback immediately after (4) and before an attempt to write at any other offset via mmap address (which implies we are yet to get a pagefault and do a get_block) what we would have is the page which is dirty have first block allocated and the other three buffer_heads unmapped. In the above case the writepage should go ahead and try to write the first blocks and clear the page_dirty flag. Further attempts to write to the page will again create a fault and result in allocating blocks and marking page dirty. If we don't write any other offset via mmap address we would still have written the first block to the disk and rest of the space will be considered as a hole. So to address this, we change all of the places where we look for delayed, unmapped, or unwritten buffer heads, and only check for delayed or unwritten buffer heads instead. Signed-off-by:Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Acked-by: