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pgo_get() vs PG_BUSY
On 07/12/24(Sat) 13:11, Mark Kettenis wrote:
> > Date: Tue, 3 Dec 2024 09:46:20 +0100
> > From: Martin Pieuchot <mpi@grenadille.net>
> >
> > Diff below changes the behavior of the pgo_get() interface with PGO_LOCKED
> > and how the page currently fetched via uvm_fault_lower_io() is released.
> >
> > The pgo_get() w/ PGO_LOCKED implementations are always called with
> > `uobj->vmobjlock' locked and the lock is never released while manipulating
> > the pages. So there is not need to set the PG_BUSY flag on the pages.
> > This is what the uao_get() and uvn_get() chunks are about.
> >
> > The chunks in uvm_fault_lower_lookup() are simplifications related to
> > the fact that none of the pgo_get() set the PG_BUSY flags.
> >
> > The rest of the diff deals with `uobjpage', the physical page fetched
> > with pgo_get() w/ PGO_SYNCIO. Instead of releasing the pages in
> > multiple places release it as soon as we've got the associated lock in
> > uvm_fault_lower_io(). The reasoning is the same as above.
> >
> > This diff is the complicated part of the current parallelisation I'm
> > working on, so do not hesitate if you have any question.
> >
> > ok?
>
> This seems to leave behind a comment that states that pgo_get() will
> set the PG_BUSY flag in uvm_fault_lower_lookup():
>
> /*
> * if center page is resident and not
> * PG_BUSY, then pgo_get made it PG_BUSY
> * for us and gave us a handle to it.
> * remember this page as "uobjpage."
> * (for later use).
> */
>
> That needs to be fixed.
Indeed, updated diff below with fixed comments.
> [...]
> > @@ -1324,21 +1323,6 @@ uvm_fault_lower(struct uvm_faultinfo *ufi, struct uvm_faultctx *flt,
> > * out of memory resources?
> > */
> > if (anon == NULL || pg == NULL) {
> > - /*
> > - * arg! must unbusy our page and fail or sleep.
> > - */
> > - if (uobjpage != PGO_DONTCARE) {
> > - uvm_lock_pageq();
> > - uvm_pageactivate(uobjpage);
> > - uvm_unlock_pageq();
>
> Shouldn't we still activate the page in this case?
Yes we should. We do it in uvm_fault_lower_io().
Index: uvm/uvm_aobj.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_aobj.c,v
diff -u -p -r1.111 uvm_aobj.c
--- uvm/uvm_aobj.c 27 Nov 2024 10:41:38 -0000 1.111
+++ uvm/uvm_aobj.c 9 Dec 2024 09:38:43 -0000
@@ -1038,8 +1038,6 @@ uao_get(struct uvm_object *uobj, voff_t
/*
* useful page: plug it in our result array
*/
- atomic_setbits_int(&ptmp->pg_flags, PG_BUSY);
- UVM_PAGE_OWN(ptmp, "uao_get1");
pps[lcv] = ptmp;
gotpages++;
}
Index: uvm/uvm_fault.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_fault.c,v
diff -u -p -r1.152 uvm_fault.c
--- uvm/uvm_fault.c 4 Dec 2024 09:21:06 -0000 1.152
+++ uvm/uvm_fault.c 9 Dec 2024 09:43:33 -0000
@@ -1147,14 +1147,13 @@ uvm_fault_lower_lookup(
pages[lcv] == PGO_DONTCARE)
continue;
+ KASSERT((pages[lcv]->pg_flags & PG_BUSY) == 0);
KASSERT((pages[lcv]->pg_flags & PG_RELEASED) == 0);
/*
- * if center page is resident and not
- * PG_BUSY, then pgo_get made it PG_BUSY
- * for us and gave us a handle to it.
- * remember this page as "uobjpage."
- * (for later use).
+ * if center page is resident and not PG_BUSY, then pgo_get
+ * gave us a handle to it.
+ * remember this page as "uobjpage." (for later use).
*/
if (lcv == flt->centeridx) {
uobjpage = pages[lcv];
@@ -1162,8 +1161,13 @@ uvm_fault_lower_lookup(
}
if (pmap_extract(ufi->orig_map->pmap, currva, &pa))
- goto next;
+ continue;
+ /*
+ * calling pgo_get with PGO_LOCKED returns us pages which
+ * are neither busy nor released, so we don't need to check
+ * for this. we can just directly enter the pages.
+ */
if (pages[lcv]->wire_count == 0) {
uvm_lock_pageq();
uvm_pageactivate(pages[lcv]);
@@ -1175,24 +1179,17 @@ uvm_fault_lower_lookup(
KASSERT(flt->wired == FALSE);
/*
- * Since this page isn't the page that's
- * actually faulting, ignore pmap_enter()
- * failures; it's not critical that we
+ * Since this page isn't the page that's actually faulting,
+ * ignore pmap_enter() failures; it's not critical that we
* enter these right now.
+ * NOTE: page can't be PG_WANTED or PG_RELEASED because we've
+ * held the lock the whole time we've had the handle.
*/
(void) pmap_enter(ufi->orig_map->pmap, currva,
VM_PAGE_TO_PHYS(pages[lcv]) | flt->pa_flags,
flt->enter_prot & MASK(ufi->entry), PMAP_CANFAIL);
entered++;
- /*
- * NOTE: page can't be PG_WANTED because
- * we've held the lock the whole time
- * we've had the handle.
- */
-next:
- atomic_clearbits_int(&pages[lcv]->pg_flags, PG_BUSY);
- UVM_PAGE_OWN(pages[lcv], NULL);
}
if (entered > 0)
pmap_update(ufi->orig_map->pmap);
@@ -1337,21 +1334,6 @@ uvm_fault_lower(struct uvm_faultinfo *uf
* out of memory resources?
*/
if (anon == NULL || pg == NULL) {
- /*
- * arg! must unbusy our page and fail or sleep.
- */
- if (uobjpage != PGO_DONTCARE) {
- uvm_lock_pageq();
- uvm_pageactivate(uobjpage);
- uvm_unlock_pageq();
-
- if (uobjpage->pg_flags & PG_WANTED)
- wakeup(uobjpage);
- atomic_clearbits_int(&uobjpage->pg_flags,
- PG_BUSY|PG_WANTED);
- UVM_PAGE_OWN(uobjpage, NULL);
- }
-
/* unlock and fail ... */
uvmfault_unlockall(ufi, amap, uobj);
if (anon == NULL)
@@ -1402,16 +1384,6 @@ uvm_fault_lower(struct uvm_faultinfo *uf
pmap_page_protect(uobjpage, PROT_NONE);
}
#endif
-
- /* dispose of uobjpage. drop handle to uobj as well. */
- if (uobjpage->pg_flags & PG_WANTED)
- wakeup(uobjpage);
- atomic_clearbits_int(&uobjpage->pg_flags,
- PG_BUSY|PG_WANTED);
- UVM_PAGE_OWN(uobjpage, NULL);
- uvm_lock_pageq();
- uvm_pageactivate(uobjpage);
- uvm_unlock_pageq();
/* done with copied uobjpage. */
rw_exit(uobj->vmobjlock);
uobj = NULL;
@@ -1593,15 +1565,13 @@ uvm_fault_lower_io(
locked = FALSE;
}
- /* didn't get the lock? release the page and retry. */
- if (locked == FALSE && pg != PGO_DONTCARE) {
+ /* release the page now, still holding object lock */
+ if (pg != PGO_DONTCARE) {
uvm_lock_pageq();
- /* make sure it is in queues */
uvm_pageactivate(pg);
uvm_unlock_pageq();
if (pg->pg_flags & PG_WANTED)
- /* still holding object lock */
wakeup(pg);
atomic_clearbits_int(&pg->pg_flags, PG_BUSY|PG_WANTED);
UVM_PAGE_OWN(pg, NULL);
@@ -1614,7 +1584,8 @@ uvm_fault_lower_io(
}
/*
- * we have the data in pg which is PG_BUSY
+ * we have the data in pg. we are holding object lock (so the page
+ * can't be released on us).
*/
*ruobj = uobj;
*ruobjpage = pg;
Index: uvm/uvm_vnode.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_vnode.c,v
diff -u -p -r1.135 uvm_vnode.c
--- uvm/uvm_vnode.c 27 Nov 2024 10:41:38 -0000 1.135
+++ uvm/uvm_vnode.c 9 Dec 2024 09:38:43 -0000
@@ -992,8 +992,6 @@ uvn_get(struct uvm_object *uobj, voff_t
* useful page: busy it and plug it in our
* result array
*/
- atomic_setbits_int(&ptmp->pg_flags, PG_BUSY);
- UVM_PAGE_OWN(ptmp, "uvn_get1");
pps[lcv] = ptmp;
gotpages++;
pgo_get() vs PG_BUSY