Re: [PATCH 03/11] vfs: Add better VFS support for page_mkwritewhen blocksize < pagesize

[Pavel Machek]

Hi!

> On filesystems where blocksize < pagesize the situation is more complicated.
> Think for example that blocksize = 1024, pagesize = 4096 and a process does:
>   ftruncate(fd, 0);
>   pwrite(fd, buf, 1024, 0);
>   map = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED, fd, 0);
>   map[0] = 'a';  ----> page_mkwrite() for index 0 is called
>   ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
>   fsync(fd); ----> writepage() for index 0 is called
> 
> At the moment page_mkwrite() is called, filesystem can allocate only one block
> for the page because i_size == 1024. Otherwise it would create blocks beyond
> i_size which is generally undesirable. But later at writepage() time, we would
> like to have blocks allocated for the whole page (and in principle we have to
> allocate them because user could have filled the page with data after the
> second ftruncate()). This patch introduces a framework which allows filesystems
> to handle this with a reasonable effort.

What happens when you do above sequence on today's kernels? Oops? 3000
bytes of random junk in file? ...?


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/