Re: IMA: kernel reading files opened with O_DIRECT

Dmitry Kasatkin
2014-07-02T18:12:07Z

Hi Jeff,

Thanks for reply.

On 2 July 2014 18:55, Jeff Moyer <jmoyer@redhat.com> wrote:
> Hi, Dmitry,
>
> Dmitry Kasatkin <d.kasatkin@samsung.com> writes:
>
>> Hi,
>>
>> We are looking for advice on reading files opened for direct_io.
>
> [snip]
>
>> 2. Temporarily clear O_DIRECT in file->f_flags.
>
> [snip]
>
>> 3. Open another instance of the file with 'dentry_open'
>
> [snip]
>
>> Is temporarily clearing O_DIRECT flag really unacceptable or not?
>
> It's acceptable. However, what you're proposing to do is read the
> entire file into the page cache to calculate your checksum. Then, when
> the application goes to read the file using O_DIRECT, it will ignore the
> cached copy and re-read the portions of the file it wants from disk. So
> yes, you can do that, but it's not going to be fast. If you want to
> avoid polluting the cache, you can call invalidate_inode_pages2_range
> after you're done calculating your checksum.
>

Ok. If I understand correctly, after reading chunck/range like
kernel_read(offset, len),
just always drop loaded pages like

invalidate_inode_pages2_range(inode->i_mapping, offset, offset + len);

I see that generic_file_direct_write() calls this function too, before
doing direct IO and after...

Thanks!

>> Or may be there is a way to allocate "special" user-space like buffer
>> for kernel and use it with O_DIRECT?
>
> In-kernel O_DIRECT support has been proposed in the past, but there is
> no solution for that yet.
>
> Cheers,
> Jeff

--
Thanks,
Dmitry