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pgo_get() vs PG_BUSY
> 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?
> 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