For unlinked files, do not msync(2) or sync on inactivation.

[kib]

Summary

One consequence of the patch is that msyncing unlinked file mappings no longer reduces the amount of the dirty memory in the system, but I do not think that there are users of msync(2) that utilize it for such side-effect.

The patch is reported to significantly help with the firefox performance.

PR: 222356

[alc]

As long as the page daemon will still launder the pages when we are short of memory, I don't see a problem with msync() ignoring them.

[kib]

In D12411#257518, @alc wrote:

As long as the page daemon will still launder the pages when we are short of memory, I don't see a problem with msync() ignoring them.

Yes, this only prevents syncing on explicit msync(2), and on the vnode inactivation. Since the inactivation for unlinked UFS vnode triggers reclamation, the writes there are discarded immediately after finished.

[alc]

sys/kern/vfs_subr.c

3058 ↗

(On Diff #33216)

Since "obj" is no longer used unconditionally, I would write "(obj = vp->v_object) != NULL" here.