[petsc-dev] DM not propagating into Fieldsplit when using external DMShell?

[Boris Boutkov]

Hello all,

I am trying to solve a Stokes type problem using PETSc's Fieldsplit
infrastructure while providing external GMG information coming from libMesh
and am running into issues when using the two simultaneously.

I have verified that FS + MG behaves as expected on its own, (I can use
-pc_type gamg on the velocity block just fine), and I also checked that the
construction of the external DMShell hierarchy also behaves as expected
when using -pc_type mg in the absence of fieldsplit; yet when I combine the
two I get errors indicating that "This DM cannot coarsen" in dm.c.

After a little digging it appeared to me that during
PCApply_FieldSplit_Schur the DM which I provided to SNES that is attached
to the PC isn't propagating through to the sub PC/KSPs because while
inspecting the pc->dm data I could see my coarsen/refine hooks which I
provided, while the sub PCs and KSPs seemed to lack those pointers.

I tried a naive fix by preempting the schurfactorization switch in
PCApply_FieldSplit_Schur to try and extract the DM I provided and push it
through to the other Schur components ala:

ierr = PCGetDM(pc,&dm);CHKERRQ(ierr);
ierr = PCSetDM(kspA->pc,dm);CHKERRQ(ierr);
ierr = KSPSetDM(kspA,dm);CHKERRQ(ierr);

and likewise for the Upper and Lower PC/KSPs, but with such a change I
still seem to lose the contexts which I attached to my DMs during the GMG
setup phase, which in turn prevents PCSetUp_MG from completing successfully.

As such, I'm wondering if there are any additional setup steps which I am
forgetting to take into account here, or if there's something else I could
try to accommodate this solver configuration.

Thanks as always for any assistance,

- Boris Boutkov
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