On Tue, Nov 21 2017, Mike Snitzer wrote: > On Mon, Nov 20 2017 at 8:35pm -0500, > Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > >> On Mon, Nov 20 2017 at 7:34pm -0500, >> NeilBrown <neilb@xxxxxxxx> wrote: >> >> > On Mon, Nov 20 2017, Mike Snitzer wrote: >> > >> > > >> > > But I've now queued this patch for once Linus gets back (reverts DM >> > > changes from commit 47e0fb461f): >> > > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c9fdc42ba23eabd1ba7aef199fb9bb4b4fe5c545 >> > >> > This patch does two things. >> > 1/ It removes the BIOSET_NEED_RESCUER flag from biosets created by dm. >> > This a functional changed over the code from before my patches. >> > Previously, all biosets were given a rescuer thread. >> > After my patch set, biosets only got a rescuer thread if >> > BIOSET_NEED_RESCUER was passed, and it was passed for all biosets. >> > I then removed it from places were I was certain it wasn't needed. >> > I didn't remove it from dm because I wasn't certain. Your >> > patch does remove the flags, which I think is incorrect - see below. > > Yeap, definitely was incorrect. I've dropped the patch. > >> > 2/ It changes flush_current_bio_list() so that bios allocated from a >> > bioset that does not have a rescue_workqueue are now added to >> > the ->rescue_list for their bio_set, and ->rescue_work is queued >> > on the NULL ->rescue_workqueue, resulting in a NULL dereference. >> > I suspect you don't want this. > > Yes, I see that now. > >> > The patch description claims that the patch fixes something, but it >> > isn't clear to me what it is meant to be fixing. >> > >> > It makes reference to dbba42d8 which is described as removing an unused >> > bioset process, though what it actually does is remove an used bioset >> > (and obvious the process disappears with it). My patch doesn't change >> > that behavior. >> >> Well I looked at this because Zdenek reported that with more recent >> kernels he is seeing the "bioset" per DM device again (whereas it was >> thought to be removed with mikulas' commit dbba42d8 -- but that commit >> removed "bioset" only in terms of q->bio_split. > > I think Zdenek triggered a false-positive that DM had magically sprouted > a new "bioset" rescue_workqueue. Reality is I cannot see how each > bio-based DM device can avoid having one. And the commit d67a5f4b59 > ("dm: flush queued bios when process blocks to avoid deadlock") I > referenced earlier very much makes DM depend on it even more. > > So apologies for being so off-base (by looking to prematurely revert > DM's use of BIOSET_NEED_RESCUER, etc). > >> > Please see >> > https://www.redhat.com/archives/dm-devel/2017-August/msg00310.html > > I'll very likely pick these up for 4.16 shortly. But hope to work > through complete removal of DM's use of BIOSET_NEED_RESCUER for 4.16 as > well. > >> > and >> > https://www.redhat.com/archives/dm-devel/2017-August/msg00315.html > > This one [1] needs a lot of review and testing. Particularly against this > test case that Mikulas created to reproduce the snapshot deadlock (same > deadlock that motivated commit dbba42d8): > https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html Thanks for that link. I'll try to make time to experiment with the test code and confirm my proposed approach doesn't break it. > >> > for which the thread continues: >> > https://www.redhat.com/archives/dm-devel/2017-September/msg00001.html > > Wish I could clone myself (or Kent, the world needs 2 Kents!) and pursue > this: https://www.redhat.com/archives/dm-devel/2014-May/msg00100.html In that email Kent mentions "punt off to a per request_queue workqueue". That "per request_queue workqueue" is what I'm trying to get rid of. I don't think this is a good direction. > > Short of that, how would you like to proceed? I'd like to confirm that my approach 1/ doesn't re-introduce a deadlock 2/ doesn't hurt performance and then merge it. Though to be honest, I don't recall exactly what "my approach" is. Your next email picks out two important patches which probably cover it. If/when I get to do the testing I'll let you know how it goes. Thanks, NeilBrown > >> > That would then just leave bcache.... I find it a bit of a challenge to >> > reason about the code in bcache, but if we can remove >> > BIOSET_NEED_RESCUER from dm, that will be an extra incentive for me to learn :-) >> >> I'm all for properly removing BIOSET_NEED_RESCUER from DM. > > Should we work to make [1] (above) sure it fixes Mikulas' test case? > > I'll set in on reviewing and playing with [1] now. > > Thanks, > Mike > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel
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