On Sat, Dec 04, 2010 at 01:43:08AM -0500, Mike Snitzer wrote: > I'm late to this old thread but I stumbled across it while auditing the > various dm-devel patchwork patches, e.g.: > https://patchwork.kernel.org/patch/83666/ > https://patchwork.kernel.org/patch/83932/ > > On Mon, Mar 08 2010 at 8:14am -0500, > Lars Ellenberg <lars.ellenberg@xxxxxxxxxx> wrote: > > > On Mon, Mar 08, 2010 at 03:35:37AM -0500, Mikulas Patocka wrote: > > > Hi > > > > > > That patch with limits->max_segments = 1; is wrong. It fixes this bug > > > sometimes and sometimes not. > > > > > > The problem is, if someone attempts to create a bio with two vector > > > entries, the first maps the last sector contained in some page and the > > > second maps the first sector of the next physical page: it has one > > > segment, it has size <= PAGE_SIZE, but it still may cross raid stripe and > > > the raid driver will reject it. > > > > Now that you put it that way ;) > > You are right. > > > > My asumption that "single segment" was > > equalvalent in practice with "single bvec" > > does not hold true in that case. > > > > Then, what about adding seg_boundary_mask restrictions as well? > > max_sectors = PAGE_SIZE >> 9; > > max_segments = 1; > > seg_boundary_mask = PAGE_SIZE -1; > > or some such. > > > > > > > This is not the first time this has been patched, btw. > > > > > See https://bugzilla.redhat.com/show_bug.cgi?id=440093 > > > > > and the patch by Mikulas: > > > > > https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff > > > > > > Look at this patch, it is the proper way how to fix it: create a > > > merge_bvec_fn that reject more than one biovec entry. > > > > If adding seg_boundary_mask is still not sufficient, > > lets merge that patch instead? > > Why has it been dropped, respectively never been merged? > > It became obsolete for dm-linear by 7bc3447b, > > but in general the bug is still there, or am I missing something? > > No it _should_ be fixed in general given DM's dm_merge_bvec() _but_ I > did uncover what I think is a subtle oversight in its implementation. > > Given dm_set_device_limits() sets q->limits->max_sectors, > shouldn't dm_merge_bvec() be using queue_max_sectors rather than > queue_max_hw_sectors? > > blk_queue_max_hw_sectors() establishes that max_hw_sectors is the hard > limit and max_sectors the soft. But AFAICT no relation is maintained > between the two over time (even though max_sectors <= max_hw_sectors > _should_ be enforced; in practice there is no blk_queue_max_sectors > setter that uniformly enforces as much). Just for the record, in case someone finds this in the archives, and wants to backport or base his own work on this: A long time ago, there was no .max_hw_sectors. Then max_hw_sectors got introduced, but without accessor function. Before 2.6.31, there was no blk_queue_max_hw_sectors(), only blk_queue_max_sectors(), which set both. 2.6.31 introduced some blk_queue_max_hw_sectors(), which _only_ set max_hw_sectors, and enforced a lower limit of BLK_DEF_MAX_SECTORS, so using that only, you have not been able to actually set lower limits than 512 kB. With 2.6.31 to 2.6.33, inclusive, you still need to use blk_queue_max_sectors() to set your limits. 2.6.34 finally dropped the newly introduced function again, but renamed the other, so starting with 2.6.34 you need to use blk_queue_max_hw_sectors(), which now basically has the function body blk_queue_max_sectors() had up until 2.6.33. > dm_set_device_limits() will set q->limits->max_sectors to <= PAGE_SIZE > if an underlying device has a merge_bvec_fn. Therefore, dm_merge_bvec() > must use queue_max_sectors() rather than queue_max_hw_sectors() to check > the appropriate limit. IMO, you should not do this. max_sectors is a user tunable, capped by max_hw_sectors. max_hw_sectors is the driver limit. Please set max_hw_sectors in dm_set_device_limits instead. BTW, e.g. o_direct will adhere to max_hw_limits, but happily ignore max_sectors, I think. > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> > --- > drivers/md/dm.c | 5 ++--- > 1 files changed, 2 insertions(+), 3 deletions(-) > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c > index 7cb1352..e83dcc8 100644 > --- a/drivers/md/dm.c > +++ b/drivers/md/dm.c > @@ -1358,12 +1358,11 @@ static int dm_merge_bvec(struct request_queue *q, > /* > * If the target doesn't support merge method and some of the devices > * provided their merge_bvec method (we know this by looking at > - * queue_max_hw_sectors), then we can't allow bios with multiple vector > + * queue_max_sectors), then we can't allow bios with multiple vector > * entries. So always set max_size to 0, and the code below allows > * just one page. > */ > - else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9) > - > + else if (queue_max_sectors(q) <= PAGE_SIZE >> 9) > max_size = 0; > > out_table: Lars -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
