exploit sparsity in swap_pager_copy

[dougm]

Summary

This is based on the observation by Yoshihiro Ota expressed in D22256, but exploits that observation differently.

Modify swp_pager_meta_free to create swp_pager_meta_copy_free that can implement swp_pager_meta_free and also do the "Transfer source to destination" work of swap_pager_copy. The benefit of this change is fewer pctrie accesses and more efficient freeing of trie elements in swap_pager_copy.

[ota_j.email.ne.jp]

I don't think this code duplication in swap_pager_copy() is a good idea.
I updated D22256 to avoid the double lookup cost from the first revision.
I'm still testing the 2nd version though.

I've also bothered with the naming of swp_pager_meta_ctl(), this function indeed looks up and then optionally removal, too. On the other hand, I don't think reducing code reuse benefit duplicating code.

[dougm]

Define swp_pager_meta_copy_free to implement swp_pager_meta_free and part of swap_pager_copy, to avoid code duplication and to exploit some of the optimizations of the meta_free code when copying.

[dougm]

Discard the changes that eliminate swap_meta_ctl. Those changes distract from the primary purpose of this change.

[pho]

With Diff 64139 I got a "panic: vm_object_madvise: page 0xfffffe0000174cf8 is not managed", which to me seems unrelated.
https://people.freebsd.org/~pho/stress/log/dougm063.txt

[kib]

sys/vm/swap_pager.c

947

How the 'represented by a resident page' part is checked ?

955

I do not think this blank comment line is useful.

2010

I think this part of the description is not longer valid.

2014

Indent of the continuation line is wrong.

2040

Again indent is wrong.

[dougm]

Fix indentation. Discard some comments.

[ota_j.email.ne.jp]

sys/vm/swap_pager.c

2013–2014

This function now either transfer to another object or free.
The source doesn't keep at all.
I don't think "copy" is appropriate.

How about swp_pager_meta_xfer / swp_pager_meta_transfer with implication of transferring to NULL is a removal?

swp_pager_meta_free() is exclusive use for free.

[dougm]

Rename "copy" to "xfer" or "transfer" in several places.
Move a dstobject==NULL test from callee to caller.

[alc]

This looks like a reasonable change.

sys/vm/swap_pager.c

2010–2011

I don't think that this sentence is true.

[dougm]

Remove an inaccurate comment.

[alc]

sys/vm/swap_pager.c

949

I don't think that the comment about the destination not being resident is guaranteed to be true.