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Add a comment describing the 'fast path' that was introduced in r270011.

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Reviewed by:	kib
MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division
This commit is contained in:
Alan Cox 2016-07-20 17:20:22 +00:00
parent d4c893910d
commit 8d67b8c863
1 changed files with 9 additions and 0 deletions
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9
sys/vm/vm_fault.c
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@ -354,6 +354,15 @@ RetryFault:;
KASSERT((fault_flags & VM_FAULT_WIRE) == 0,
("!wired && VM_FAULT_WIRE"));
/*
* Try to avoid lock contention on the top-level object through
* special-case handling of some types of page faults, specifically,
* those that are both (1) mapping an existing page from the top-
* level object and (2) not having to mark that object as containing
* dirty pages. Under these conditions, a read lock on the top-level
* object suffices, allowing multiple page faults of a similar type to
* run in parallel on the same top-level object.
*/
if (fs.vp == NULL /* avoid locked vnode leak */ &&
(fault_flags & (VM_FAULT_WIRE | VM_FAULT_DIRTY)) == 0 &&
/* avoid calling vm_object_set_writeable_dirty() */