On Mon, 05 Mar 2007 17:10:11 -0600
Dave Kleikamp <[EMAIL PROTECTED]> wrote:
> On Mon, 2007-03-05 at 14:57 -0800, Andrew Morton wrote:
> > Is OK, I think. nobh_prepare_write() brings the outside-from-and-to
> > sections of the page uptodate and memset in nobh_truncate_page() brings the
> > rest of the page uptodate.
> >
> > We bring the to->PAGE_CACHE_SIZE section uptodate twice, which could be
> > optimised.
>
> Why not have nobh_truncate_page() call prepare_write() with to =
> PAGE_CACHE_SIZE? I don't really grok:
>
> to = (offset + blocksize) & ~(blocksize - 1);
OK, let's say we have a 4k pagesize and a 1k fs blocksize and someone does
ftruncate(fd, 1024 + 100).
We need to:
- load bytes (0 ... 1024+100) from disk (these are live file data)
- zero the bytes (1024+100 ... 1024+1024) (these are data outside the
truncation point, but inside the fs block which straddles the truncation
point)
- zero the bytes (1024+1024 ... 4096) (these are the rest of the
pagecache page, outside the truncation point).
- mark the page dirty so those zeroes outside the truncation point but
inside the block which straddles i_size (ie: (1024+100 ... 1024+1024)
get written back.
Now, it just so happens that
nobh_prepare_write(NULL, page, 1024+100, 1024+1024)
will indeed load bytes (0 ... 1024+1024) from disk, and will then zero out
bytes (1024+100 ... 4096) for us (I think - assumes that ->get_block does
the right thing with buffer_mapped)). We could exploit that knowledge of
prepare_write() internals in nobh_truncate_page().
But as it stands, nobh_truncate_page() is being formal and is not making
assumptions about what nobh_prepare_write() does with blocks outside
i_size.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
