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Re-check for wirings after busying the page in vm_page_release_locked().

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A concurrent unlocked lookup can wire the page after
vm_page_release_locked() releases the last wiring, in which case
vm_page_release_locked() must not free the page.  Once the xbusy lock is
acquired, that, the object lock and the fact that the page is unmapped
ensure that the wire count cannot increase, so re-check for new wirings
after the page is xbusied.

Update the comment above vm_page_wired() to reflect the new
synchronization rules.

Reported by:	glebius
Reviewed by:	alc, jeff, kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D24592
This commit is contained in:
Mark Johnston 2020-04-28 13:51:41 +00:00
parent eb2aebeafd
commit a9ea09e548
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=360436
2 changed files with 12 additions and 3 deletions
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11
sys/vm/vm_page.c
View file

@ -4165,7 +4165,16 @@ vm_page_release_locked(vm_page_t m, int flags)
if ((flags & VPR_TRYFREE) != 0 &&
(m->object->ref_count == 0 || !pmap_page_is_mapped(m)) &&
m->dirty == 0 && vm_page_tryxbusy(m)) {
vm_page_free(m);
/*
* An unlocked lookup may have wired the page before the
* busy lock was acquired, in which case the page must
* not be freed.
*/
if (__predict_true(!vm_page_wired(m))) {
vm_page_free(m);
return;
}
vm_page_xunbusy(m);
} else {
vm_page_release_toq(m, PQ_INACTIVE, flags != 0);
}

4
sys/vm/vm_page.h
View file

@ -958,8 +958,8 @@ vm_page_drop(vm_page_t m, u_int val)
*
* Perform a racy check to determine whether a reference prevents the page
* from being reclaimable. If the page's object is locked, and the page is
* unmapped and unbusied or exclusively busied by the current thread, no
* new wirings may be created.
* unmapped and exclusively busied by the current thread, no new wirings
* may be created.
*/
static inline bool
vm_page_wired(vm_page_t m)