Re: [Dazuko-devel] DazukoFS 3.1.3-rc1 posted

[Lino Sanfilippo]

John Ogness wrote:
> - A couple mmap() write functions were implemented to hopefully fix
>   some kernel crashes reported[1].
>
> [1] http://lists.gnu.org/archive/html/dazuko-devel/2009-10/msg00002.html
>   

Hi,

the reason for these kernel crashes is the use of the 
generic_file_aio_write()
kernel api.
Using these genericXXX functions in the dazukofs file operations
is always dangerous, since they might require function definitions, that
are not provided (and not needed) by dazuko (i.e all kind of functions that
are called to physically write data to disk).

In this case generic_file_aio_write() marks the pages that belong to the
dazukofs inodes as dirty, which tells the kernel that they should be
written to disk. In turn the kernel calls the write_begin() and writepage()
functions of dazukofs address_space.
Note that these functions should never be called by the kernel, since 
dazukofs
does not write any data to disk itself.

If we want dazukofs to provide functions for async write/read, we have 
to do
an own dazukofs wrapper (see dazukofs_mmap()) that checks if the lower 
filesystem is
prepared to handle these functions and if so directs calls to the lower
filesystem.


Regards,
Lino Sanfilippo

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