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

[John Ogness]

On 2010-01-04, Lino Sanfilippo <address@hidden> wrote:
> 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).

It is dangerous only if it is not what we want (or is being used
incorrectly by us). I want to avoid copy/pasting code.

> 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.

Agreed. They should not be called. I assume we are doing something
wrong.

> 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.

I would prefer to know why generic_file_readonly_mmap() isn't working
as we expect: as read-only. Perhaps we are simply using the generic
functions incorrectly. DazukoFS is not the only filesystem that does
not support mmap-writing. Others are jffs2, afs, 9p.

John Ogness

-- 
Dazuko Maintainer