| Message ID | 20180502140359.18222-1-mreitz@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 05/02/2018 09:03 AM, Max Reitz wrote: > Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite > and pwrite_zeroes, and also add a note on when you absolutely need to > support it. > > Signed-off-by: Max Reitz <mreitz@redhat.com> > --- Thanks, that helps. Reviewed-by: Eric Blake <eblake@redhat.com> > I did not include a note on how this might be useful to protocol > drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end > up on the protocol level anyway. > (I suppose it would be possible in theory if a protocol driver reports a > certain range as unallocated and the user installs a copy-on-read > driver on top, but I think every protocol driver currently reports > everything as allocated.) It doesn't help that I still haven't revisited an audit of whether all the protocol drivers have a sane status return to begin with (fallout from Kevin's review of my byte-based block status that landed in 2.12). But that doesn't hold up this patch.
On 2018-05-02 16:03, Max Reitz wrote: > Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite > and pwrite_zeroes, and also add a note on when you absolutely need to > support it. > > Signed-off-by: Max Reitz <mreitz@redhat.com> > --- > I did not include a note on how this might be useful to protocol > drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end > up on the protocol level anyway. > (I suppose it would be possible in theory if a protocol driver reports a > certain range as unallocated and the user installs a copy-on-read > driver on top, but I think every protocol driver currently reports > everything as allocated.) > --- > include/block/block_int.h | 18 ++++++++++++++++-- > 1 file changed, 16 insertions(+), 2 deletions(-) Thanks for the review, applied to my block branch. Max
diff --git a/include/block/block_int.h b/include/block/block_int.h index e3d6219f4e..76b589da57 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -656,10 +656,24 @@ struct BlockDriverState { /* I/O Limits */ BlockLimits bl; - /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */ + /* Flags honored during pwrite (so far: BDRV_REQ_FUA, + * BDRV_REQ_WRITE_UNCHANGED). + * If a driver does not support BDRV_REQ_WRITE_UNCHANGED, those + * writes will be issued as normal writes without the flag set. + * This is important to note for drivers that do not explicitly + * request a WRITE permission for their children and instead take + * the same permissions as their parent did (this is commonly what + * block filters do). Such drivers have to be aware that the + * parent may have taken a WRITE_UNCHANGED permission only and is + * issuing such requests. Drivers either must make sure that + * these requests do not result in plain WRITE accesses (usually + * by supporting BDRV_REQ_WRITE_UNCHANGED, and then forwarding + * every incoming write request as-is, including potentially that + * flag), or they have to explicitly take the WRITE permission for + * their children. */ unsigned int supported_write_flags; /* Flags honored during pwrite_zeroes (so far: BDRV_REQ_FUA, - * BDRV_REQ_MAY_UNMAP) */ + * BDRV_REQ_MAY_UNMAP, BDRV_REQ_WRITE_UNCHANGED) */ unsigned int supported_zero_flags; /* the following member gives a name to every node on the bs graph. */
Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite and pwrite_zeroes, and also add a note on when you absolutely need to support it. Signed-off-by: Max Reitz <mreitz@redhat.com> --- I did not include a note on how this might be useful to protocol drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end up on the protocol level anyway. (I suppose it would be possible in theory if a protocol driver reports a certain range as unallocated and the user installs a copy-on-read driver on top, but I think every protocol driver currently reports everything as allocated.) --- include/block/block_int.h | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)