Details

Reviewers

dougm
alc
kib

Commits

rG5d32157d4e5c: vm_object: Modify vm_object_allocate_anon() to return OBJT_SWAP objects

Summary

With this change, OBJT_DEFAULT objects are no longer allocated.
Instead, anonymous objects are always of type OBJT_SWAP and always have
OBJ_SWAP set.

Modify the page fault handler to check the swap block radix tree in
places where it checked for objects of type OBJT_DEFAULT. In
particular, there's no need to invoke getpages for an anonymous object
with no swap blocks assigned. It might be that we could relax this to
non-anonymous objects as well (e.g., we could perhaps avoid needlessly
calling getpages during faults on mappings of tmpfs files).

Diff Detail

Repository

rS FreeBSD src repository - subversion

Lint

Lint Passed

Unit

No Test Coverage

Build Status

Buildable 46405
Build 43294: arc lint + arc unit

Event Timeline

markj created this revision.Jul 12 2022, 4:22 PM

Herald added a subscriber: imp. · View Herald TranscriptJul 12 2022, 4:22 PM

markj requested review of this revision.Jul 12 2022, 4:22 PM

Harbormaster completed remote builds in B46382: Diff 108057.Jul 12 2022, 4:22 PM

markj added a child revision: D35786: swap_pager: Removing handling for objects with OBJ_SWAP clear.Jul 12 2022, 4:22 PM

kib added inline comments.Jul 12 2022, 4:49 PM

sys/vm/vm_fault.c
235	Why do you require OBJ_ANON there?

markj added inline comments.Jul 12 2022, 5:18 PM

sys/vm/vm_fault.c
235	I think it is not strictly required. Just being conservative since the fault handler previously only looked for OBJT_DEFAULT objects, and those are almost always anonymous objects. I think I would relax this check in a separate commit, if at all.

kib accepted this revision.Jul 12 2022, 6:08 PM

kib added inline comments.

sys/vm/vm_fault.c
235	This is not strictly equivalent to the old condition, anyway. I see no reason to keep OBJ_ANON.

This revision is now accepted and ready to land.Jul 12 2022, 6:08 PM

In _vm_object_allocate(), initialize swp.writemappings as we do the swap block
tree.

This revision now requires review to proceed.Jul 13 2022, 1:24 PM

Harbormaster completed remote builds in B46405: Diff 108099.Jul 13 2022, 1:24 PM

markj mentioned this in D35786: swap_pager: Removing handling for objects with OBJ_SWAP clear.Jul 13 2022, 1:26 PM

alc added inline comments.Jul 13 2022, 3:19 PM

sys/vm/vm_fault.c
227	I would replace "does" by "can" because a copy is not certain to exist even if this function returns true.
235	I found the inclusion of OBJ_ANON here to be unexpected and confusing; in other words, it forced me to think. So, I would also argue for removing it.