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pgo_get() vs PG_BUSY
> Date: Tue, 17 Dec 2024 10:35:36 +0100
> From: Martin Pieuchot <mpi@grenadille.net>
>
> On 16/12/24(Mon) 21:07, Mark Kettenis wrote:
> > > Date: Mon, 9 Dec 2024 10:49:56 +0100
> > > From: Martin Pieuchot <mpi@grenadille.net>
> > >
> > > 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.
> >
> > Great!
> >
> > > > [...]
> > > > > @@ -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().
> >
> > Hmm, but if we got the page through uvm_fault_lower_lookup() it
> > doesn't happen since uvm_fault_lower_lookup() does an early continue
> > for the center page. Or is that a corner cadse we don't really care
> > about?
>
> I can't say if we don't really care or not. This activate disappeared
> in NetBSD and I'd rather keep it for the moment. Thanks for spotting
> it.
>
> Updated diff below.
ok kettenis@
> 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 17 Dec 2024 09:04:52 -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 -r1.153 uvm_fault.c
> --- uvm/uvm_fault.c 15 Dec 2024 11:02:59 -0000 1.153
> +++ uvm/uvm_fault.c 17 Dec 2024 09:31:57 -0000
> @@ -1140,14 +1140,13 @@ 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];
> @@ -1155,8 +1154,13 @@ 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]);
> @@ -1168,24 +1172,17 @@ 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);
> @@ -1273,6 +1270,11 @@ uvm_fault_lower(struct uvm_faultinfo *uf
> if (uobjpage) {
> /* update rusage counters */
> curproc->p_ru.ru_minflt++;
> + if (uobjpage != PGO_DONTCARE) {
> + uvm_lock_pageq();
> + uvm_pageactivate(uobjpage);
> + uvm_unlock_pageq();
> + }
> } else {
> error = uvm_fault_lower_io(ufi, flt, &uobj, &uobjpage);
> if (error != 0)
> @@ -1330,21 +1332,6 @@ 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)
> @@ -1395,16 +1382,6 @@ 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;
> @@ -1586,15 +1563,13 @@ 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);
> @@ -1607,7 +1582,8 @@ 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 17 Dec 2024 09:04:52 -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