Re: [PATCH v5 06/10] xfs: add XFS_IOC_FREE_EOFBLOCKS ioctl

Brian Foster
2012-10-24T16:13:32Z

On 10/22/2012 09:31 PM, Dave Chinner wrote:
> On Fri, Oct 05, 2012 at 10:17:12AM -0400, Brian Foster wrote:
...
>> /*
>> + * Speculative preallocation trimming.
>> + */
>> +#define XFS_EOFBLOCKS_VERSION 1
>> +struct xfs_eofblocks {
>> + __u32 eof_version;
>> + __u32 eof_flags;
>> + unsigned char pad[12];
>> +};
>
> 12 bytes of padding is a bit wierd at this point. It's
> problematic for 32bit userspace on 64 bit kernels, in that the size
> of the structure can end up different (i.e. 20 bytes on 32b, 24 bytes
> on 64b) depending on the architectures natural alignment.
>
> I can also see that adding multiple extra variables to the structure
> are quite likely (e.g. per-ag control, start/end inode numbers,
> etc), so 12 bytes of padding really isn't sufficient, IMO. I'd tend
> to pad out to, say, 128 bytes rather than 32, just in case. i.e:
>
> __u64 pad[15];
>
> And then take away from this padding space as you add functioanlity
> in fucture patches.
>
> This extra padding means the version number won't need to increase
> any time soon, as it will be a while before we run out of either
> padding or flag space, instead of as soon as we add a new function
> to the ioctl....
>

Ok, I'll convert the file size parameter to a __u64 (as per discussed in
the spaceman eofblocks command thread) and expand the padding.

>> +
>> +/* eof_flags values */
>> +#define XFS_EOF_FLAGS_SYNC 0x01 /* sync/wait mode scan */
>
> I kind of prefer flags being defined by (1 << 0) style to keep
> larger flag numbers concise, but that's not a big deal.
>

Eh, I find this cleaner than looking at the raw values as well.

>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
>> index 0e0232c..ad4352f 100644
>> --- a/fs/xfs/xfs_ioctl.c
>> +++ b/fs/xfs/xfs_ioctl.c
>> @@ -42,6 +42,7 @@
>> #include "xfs_inode_item.h"
>> #include "xfs_export.h"
>> #include "xfs_trace.h"
>> +#include "xfs_icache.h"
>>
>> #include <linux/capability.h>
>> #include <linux/dcache.h>
>> @@ -1602,6 +1603,21 @@ xfs_file_ioctl(
>> error = xfs_errortag_clearall(mp, 1);
>> return -error;
>>
>> + case XFS_IOC_FREE_EOFBLOCKS: {
>> + struct xfs_eofblocks eofb;
>> + int flags;
>> +
>> + if (copy_from_user(&eofb, arg, sizeof(eofb)))
>> + return -XFS_ERROR(EFAULT);
>> +
>> + if (eofb.eof_version != XFS_EOFBLOCKS_VERSION)
>> + return -XFS_ERROR(EINVAL);
>
> Checking that no unsupported flags are set here is necessary. Also,
> checking the padding is zero is probably a good idea, as it will
> force applications to zero the structure properly before being able
> to use this interface properly....
>

Ok.

>> + flags = (eofb.eof_flags & XFS_EOF_FLAGS_SYNC) ? SYNC_WAIT : SYNC_TRYLOCK;
>
> Line if a bit too long. However, would it be better to place this
> inside xfs_icache_free_eofblocks()?
>

Are you suggesting to eliminate the flags parameter to
xfs_icache_free_eofblocks? The reason for the current interface is that
the background scan caller doesn't require the eofb parameter, so I
decided to generalize the sync/wait parameter in the caller.

If we want to push it into xfs_icache_free_eofblocks(), I think it would
be better at that point to eliminate the flags param and either infer
SYNC_TRYLOCK on a NULL eofb or to require an eofb and pass one with
a cleared XFS_EOF_FLAGS_SYNC from the background scan. Thoughts?

Brian

>> + error = xfs_icache_free_eofblocks(mp, flags, &eofb);
>
> Cheers,
>
> Dave.