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pgo_get() vs PG_BUSY
> Date: Sun, 15 Dec 2024 12:18:13 +0100
> From: Martin Pieuchot <mpi@grenadille.net>
>
> On 09/12/24(Mon) 10:49, Martin Pieuchot wrote:
> > 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().
>
> Any ok?
Concerts are done, so I hope to have some time to review your reply
tomorrow.
> > 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